We don't have to find a definite reason for scribal interpolation to conclude a certain phrase is an interpolation. The internal evidence from consistency and common sense point of view is sufficient to conjecture about it. In this case the more authentic family of manuscripts help us to rule this out as fake. We can only speculate on the reason behind it, such as the scribe was drunk or sleepy, or added it from the margin glosses, added by himself as a gloss.
NET Bible notes: John 3:13: “No one has ascended into heaven except the one who descended from heaven — the Son of Man.”
3:13  tc Most witnesses, including a few important ones (A[*] Θ Ψ 050 Ë1,13 Ï latt syc,p,h), have at the end of this verse “the one who is in heaven” (ὁ ὢν ἐν τῷ οὐρανῷ, Jo wn en tw ouranw). A few others have variations on this phrase, such as “who was in heaven” (e syc), or “the one who is from heaven” (0141 pc sys). The witnesses normally considered the best, along with several others, lack the phrase in its entirety (Ì66,75 א B L T Ws 083 086 33 1241 pc co). On the one hand, if the reading ὁ ὢν ἐν τῷ οὐρανῷ is authentic it may suggest that while Jesus was speaking to Nicodemus he spoke of himself as in heaven even while he was on earth. If that is the case, one could see why variations from this hard saying arose: “who was in heaven,” “the one who is from heaven,” and omission of the clause. At the same time, such a saying could be interpreted (though with difficulty) as part of the narrator’s comments rather than Jesus’ statement to Nicodemus, alleviating the problem. And if v. 13 was viewed in early times as the evangelist’s statement, “the one who is in heaven” could have crept into the text through a marginal note. Other internal evidence suggests that this saying may be authentic. The adjectival participle, ὁ ὤν, is used in the Fourth Gospel more than any other NT book (though the Apocalypse comes in a close second), and frequently with reference to Jesus (1:18; 6:46; 8:47). It may be looking back to the LXX of Exod 3:14 (ἐγώ εἰμι ὁ ὤν). Especially since this exact construction is not necessary to communicate the location of the Son of Man, its presence in many witnesses here may suggest authenticity. Further, John uses the singular of οὐρανός (ourano", “heaven”) in all 18 instances of the word in this Gospel, and all but twice with the article (only 1:32 and 6:58 are anarthrous, and even in the latter there is significant testimony to the article). At the same time, the witnesses that lack this clause are very weighty and must not be discounted. Generally speaking, if other factors are equal, the reading of such mss should be preferred. And internally, it could be argued that ὁ ὤν is the most concise way to speak of the Son of Man in heaven at that time (without the participle the point would be more ambiguous). Further, the articular singular οὐρανός is already used twice in this verse, thus sufficiently prompting scribes to add the same in the longer reading. This combination of factors suggests that ὁ ὢν ἐν τῷ οὐρανῷ is not a genuine Johannism. Further intrinsic evidence against the longer reading relates to the evangelist’s purposes: If he intended v. 13 to be his own comments rather than Jesus’ statement, his switch back to Jesus’ words in v. 14 (for the lifting up of the Son of Man is still seen as in the future) seems inexplicable. The reading “who is in heaven” thus seems to be too hard. All things considered, as intriguing as the longer reading is, it seems almost surely to have been a marginal gloss added inadvertently to the text in the process of transmission. For an argument in favor of the longer reading, see David Alan Black, “The Text of John 3:13,” GTJ 6 (1985): 49-66.sn See the note on the title Son of Man in 1:51.