If we compare those three letters with those of Rom 1:8–10; 1 Cor 1:4–6; 2 Cor 1:3–7; Eph 1:1–10; Phil 1:4–6; Col 1:3–4; 1 Thess 1:2–3; 2 Thess 1:3–4; 2 Tim 1:3–4, we find that it doesn't begin with a blessing. Why is this so?
In Rom 1:8-10, Paul's blessing refers to the strong faith of the Romans; in 1 Cor 1:4-6 likewise, as Paul thanks God for their faith; 2 Cor 1:2-7 differs only in that the blessing is in the form of words of comfort; 1 Thess 2-4 is again gives thanks for their faith. Compare this to Galatians, where Paul wishes the Galatians well (Gal 1:2) but omits the customary reference to the strength of their faith. Instead, (Gal 1:6-7) Paul marvels that the Galatians have been so easily led astray and perverted the gospel of Christ. In a nutshell, Paul can not give thanks for the faith of the Galatians because they have begun to follow another preacher with a different message about Jesus. Much of this epistle is given to Paul's assertion of his own authority and chastising the Galatians for listening to other missionaries.
The formal blessing is also missing from 1 Timothy and Titus, but 1 Timothy has an abbreviated statement in 1 Tim 1:2, Titus has Paul call Titus his own son after the faith, and 2 Timothy does have Paul expressing his love for Timothy and noting Timothy's faith. I regard the three Pastorals (1 Timothy, 2 Timothy and Titus) as pseudo-Pauline epistles from the second century, although each made efforts to copy what they regarded as Paul's style.
I don't think a "concrete," "certain," answer can be given ...
However, perhaps there is a plausible explanation given the period, and given cultures at work:
Titus, Timothy, Galatians, had notable issues regarding Gentile Christians and the controversy of Pharasaic/Rabbinic doctrines and traditions being taught in the Churches.
In view of this, it would be easy to conclude that a formulated "Blessing"/Brakhah would be appropriate in the context of letters written to Hebrew believers.
On the other-hand, this formulated blessing would be misplaced among Gentile believers, and inappropriate--as they probably felt a great deal of resentment towards those concepts in general.
Perhaps Paul was simply being all things to all people :
1 Corinthians 9:19-23 - New American Standard Bible (NASB)
19 For though I am free from all men, I have made myself a slave to all, so that I may win more. 20 To the Jews I became as a Jew, so that I might win Jews; to those who are under the Law, as under the Law though not being myself under the Law, so that I might win those who are under the Law; 21 to those who are without law, as without law, though not being without the law of God but under the law of Christ, so that I might win those who are without law. 22 To the weak I became weak, that I might win the weak; I have become all things to all men, so that I may by all means save some. 23 I do all things for the sake of the gospel, so that I may become a fellow partaker of it.
Certainly it is the case that the other Epistles had Jewish and Gentile audiences. However, I am just submitting the probability of Paul/Luke/someone altering the format of a letter to fit a particular audience, and need, and knowingly avoided the possibility of placing stumbling blocks.
Further, in arguments of Primacy regarding which language the Epistles were originally written in, it may be the case that some were written in Aramaic, or Greek, or perhaps both side by side. To Aramaic audiences, perhaps the formulated blessings would be more appropriate.
These are just Hypotheses. But this simple fact remains: the format of the epistles changed, for whatever reason. This provides a Scriptural basis to depart from certain "forms" and "liturgies" in service of a particular need.
I'm not sure of its significance, but I would imagine that part of it may be to do with what was happening at the time. Galatians is believed to have been his second letter, when he was ministering in about 49AD, while Titus and 1 Timothy were in about 63AD, post-Acts, wherein he was able to see his martyring coming, and was associated with the brevity in light of little time to write, yet with such significant purpose to the content of the letters.
Galatians may have been out of character, but perhaps because it was intentionally written to seek a broader audience, he didn't provide a direct salutation.
So my reasons are split because of the differences: Galatians due to a broad audience. Titus and 1 Timothy due to brevity.