It was present originally, but accidentally dropped out -- although this scribal/mechanical explanation is not often seriously considered these days.

It was omitted to delimit the list to ten (or twelve) cases, to match the decalogue, but as Ziskind notes, there was no need to sacrifice a father-daughter clause to meet this aim.

Some have suggested that the explicit prohibition is omitted, since it is already implicit in 18:17 (as also argued by an existing answer), but (as Ziskind notes), this only "begs the question", since more distant relations are explicit, and (as Meacham notes), father-daughter incest is the most common form of incest.

Joosten, supplemented by Luciani, deepens and extends this line of thought: the omission is rhetorically motivated, and in line with rhetorical conventions, the non-mention takes on enhanced persuasiveness.

Comparison with some law codes from the ancient Near East prompts the suggestion that the Levitical code is dealing with the same set of cases found in the wider ANE legal tradition -- yet this overlooks cases where father-daughter incest is explicitly proscribed (especially the Hittite laws, see Meacham, pp. 256-7).

The rabbis noted and discussed this omission (see Meacham), and Rashi handled it in connection with 18:10 -

The nakedness of your son's daughter or your daughter's daughter you shall not uncover their nakedness, for they are your own nakedness.

The rabbinic discussion is a more technical version of deriving the father-daughter prohibition from the existing text.

Ziskind's own suggestion is that the formulation of the prohibition in terms consistent with the rest of the cases would have resulted in implicitly undermining the paternal authority in the family, with the fear that

explicit inclusion of such a statement ... would cause a loss of credibility for his program of family law reform. He therefore chose to resolve the conflict between morality and legal principle by neither condemning incest with a daughter nor explicitly permitting it either. He simply omitted reference to it altogether. To be sure, this omission showed more political prudence than moral courage. (p. 130)

Meacham is unconvinced by Ziskind's proposal. She argues in turn that the cases explicitly covered by Lev 18 cover all the cases found in the patriarchal stories, or by meaningful extrapolation from them -- except for the father-daughter case. This, of course, does occur in Genesis 19, in the case of Lot and his daughters. But this episode gives rise to two of Israel's enemies (Moab and Ammon). So whereas the incestual incidents associated with Abraham's direct line might have been seen as having precedent (and thus permissible) are proscribed explicitly, the father-daughter relationship is already stigmatized and omitted.

Leithart offers an explanation of a very different kind (included for completeness) from an explicitly Christian framework. After carefully mapping the degrees of prohibited (and permitted) relationships, Leithart argues the omission is intentional. Why? Because otherwise God himself would be guilty of it in (e.g.) Ezekiel 16, where YHWH is adoptive father and intended husband, and Jerusalem/Zion the foundling daughter/intended bride. Leithart's prefered theological resolution is trinitarian: the tension is dissolved once one sees God as not only father and husband, but as Father and Son.

It was present originally, but accidentally dropped out -- although this scribal/mechanical explanation is not often seriously considered these days.

It was omitted to delimit the list to ten (or twelve) cases, to match the decalogue, but as Ziskind notes, there was no need to sacrifice a father-daughter clause to meet this aim.

Some have suggested that the explicit prohibition is omitted, since it is already implicit in 18:17 (as also argued by an existing answer), but (as Ziskind notes), this only "begs the question", since more distant relations are explicit, and (as Meacham notes), father-daughter incest is the most common form of incest.

Joosten, supplemented by Luciani, deepens and extends this line of thought: the omission is rhetorically motivated, and in line with rhetorical conventions, the non-mention takes on enhanced persuasiveness.

The rabbis noted and discussed this omission (see Meacham), and Rashi handled it in connection with 18:10 -

The nakedness of your son's daughter or your daughter's daughter you shall not uncover their nakedness, for they are your own nakedness.

The rabbinic discussion is a more technical version of deriving the father-daughter prohibition from the existing text.

Comparison with some law codes from the ancient Near East prompts the suggestion that the Levitical code is dealing with the same set of cases found in the wider ANE legal tradition -- yet this overlooks cases where father-daughter incest is explicitly proscribed (especially the Hittite laws, see Meacham, pp. 256-7).

Ziskind's own suggestion is that the formulation of the prohibition in terms consistent with the rest of the cases would have resulted in implicitly undermining the paternal authority in the family, with the fear that

explicit inclusion of such a statement ... would cause a loss of credibility for his program of family law reform. He therefore chose to resolve the conflict between morality and legal principle by neither condemning incest with a daughter nor explicitly permitting it either. He simply omitted reference to it altogether. To be sure, this omission showed more political prudence than moral courage. (p. 130)

Meacham is unconvinced by Ziskind's proposal. She argues in turn that the cases explicitly covered by Lev 18 cover all the cases found in the patriarchal stories, or by meaningful extrapolation from them -- except for the father-daughter case. This, of course, does occur in Genesis 19, in the case of Lot and his daughters. But this episode gives rise to two of Israel's enemies (Moab and Ammon). So whereas the incestual incidents associated with Abraham's direct line might have been seen as having precedent (and thus permissible) are proscribed explicitly, the father-daughter relationship is already stigmatized and omitted.

Leithart offers an explanation of a very different kind (included for completeness) from an explicitly Christian framework. After carefully mapping the degrees of prohibited (and permitted) relationships, Leithart argues the omission is intentional. Why? Because otherwise God himself would be guilty of it in (e.g.) Ezekiel 16, where YHWH is adoptive father and intended husband, and Jerusalem/Zion the foundling daughter/intended bride. Leithart's prefered theological resolution is trinitarian: the tension is dissolved once one sees God as not only father and husband, but as Father and Son.