Probably the main point is that most "SATB arrangements" are from hymnbooks, and these are designed primarily for congregational singing. The pitch of the melody line is determined more by what's suitable for the congregation than by what's suitable for a trained soprano section. Taking Silent Night much higher than Bb will result in top notes which are uncomfortable for many members of the congregation.
If you find an arrangement that is specifically written for a choir rather than for congregational singing, you might find that it's pitched higher. But also of course, in a choral context it's very usual for a director to adapt whatever materials are to hand, especially by choosing a different pitch from the one written. If it suits the choir better, they can sing it up a tone, or whatever.
Silent Night is also a special case in various ways. It has a large range, and is also generally sung quite quietly. When singing quietly rather than loudly, many singers find low notes become relatively easier to achieve convincingly, and high notes become relatively more difficult. This might push one in the direction of a lower pitch, while a more strident tune with a similar span might benefit from being pitched higher.
There's also an issue about solo vs choral singing. For a solo tenor, D major might indeed be a reasonable key for Silent Night. For a tenor section with many singers together, that penultimate line with the top G might be much harder to pull off convincingly. A soloist has lots of leeway in terms of timbre, volume, timing, etc, but for a choral tenor section, achieving a good blend and ensemble in a quiet high passage can be tough unless they are very good.