Heaven or heavens? Matthew 5:3

I could think of a couple possible reasons. One likely reason or influence would be that "many translations" were influenced by or were following an earlier translation such as the KJV. The KJV may have been following the earlier pre-1611 English Bibles, starting with Tyndale. Although translators or revisers are sometimes accused of making changes for the sake of change, they may actually be hesitant to make changes that they do not consider necessary or important.

Another possible reason might be that some translators could consider it a figure of speech such as "heterosis of number" ["the plural for the singular"] ["This is so put when great excellence or magnitude is denoted" (Bullinger, Figures of Speech, pp. 528-529).

By the way, the 1395 edition of Wycliffe's Bible has "the kyngdom of hevenes" at Matthew 5:3.