The full question is: Why is 1 Corinthians 14:34-35 placed after v.40 in the Greek-Latin bilingual codices D F G (Claromontanus 5th C, Augiensis 9th C, Boernerianus 9th C) and the 12th-century miniscule 88?
Metzger and the United Bible Societies textual committee
No source omits 1 Corinthians 14:34-35 so the issue is not omission but placing. The standard textual critical guidebook for Bible Society translators, Bruce Metzger ‘A Textual Commentary on the Greek New Testament’ United Bible Societies 1975 p565 records the committee rating the traditional placing “include here” as B on the A-to-D scale for confidence of the committee’s decision. The alternative placing after v.40 has not been given a rating.
Again, no manuscript (and no modern translation) omits the two verses, so the only evidence regards either the large amount of Greek manuscript evidence for placement after v33 or the variant placement in the Greek-Latin bilinguals after v40.
Further suggestions in standard commentary series:
Further expansion of the UBS committee decision is summarised in the footnotes on 1 Co14:34-35 various standard scholarly commentary series. The below is taken from the 1 Corinthians volume in the BECNT commentary, but footnotes to 1Co14:34-35 in other scholarly commentary series are similar.
“the transposition occurs in only a few closely related MSS from Northern Italy spread abroad in the Middle Ages by Irish monastics” (Niccum 1997 p254). Manuscripts D* F and G also move other texts to make a passage seem less disjointed: Rom.16:20b is placed after Rom 16:24, Rom.16:5a is placed after Rom 16:3, and Rom.16:16b is placed after Rom 16:21 – presumably to improve style.” 1 Corinthians Baker Exegetical Commentary on the New Testament. David E. Garland 2003 p676
But why? A connection with ‘their women’?
This actually doesn’t answer the question ‘why’ the Greek Latin-bilinguals displace v34-35. This is question with no certain answer, but the one suggested answer that has internal evidence from D F G to support it (and is noted by Garland and other commentaries) is shown in the next entry in Metzger’s TCGNT p565 below the entry on the displacement in the Greek Latin bilinguals.
TCGNT notes on 14:34 that the committee read ‘gunaikes’ (women) while D F G have ‘gunaikes hymon’ (wives).
This change of ‘women’ to ‘their wives’ is also present in the Latin half of the bilinguals, so D F G read:
14:39 Therefore, my brothers and sisters, be eager to prophesy, and do not forbid speaking in tongues. 40 But everything should be done in a fitting and orderly way.  Their women should remain silent in the churches. They are not allowed to speak, but must be in submission, as the law says.  If they want to inquire about something, they should ask their own men at home; for it is disgraceful for a woman to speak in the church. [Claromontanus Greek-Latin bilingual]
There is a certain logic to adding “their” with this placing, but whether it really alters the meaning of what the entire section says is debatable.