I found a video with the tenor line to an arrangement of Silent night. The tenor is found in this video:It is a low tenor line. I would even sing up to F4 or even G4 if I sang the melody as a solo piece. Is there a good reason why the tenor line is not higher? Shouldn't tenors sing higher notes in a choir?
The answer is simple: this is a low-pitched hymn arrangement in general. It's very similar to this one, though the last couple measures of the tenor line are varied. This is a very old arrangement of "Silent Night" found in many hymnals, and typically pitched in B-flat.
Note that the soprano is very low for most of the song, going down to a low B-flat (also unusual for soprano). Trying to fit alto and tenor parts below the melody (where they almost always are in hymn arrangements) means that the tenor line is necessarily lower in its range for this setting.
Also, note the general character of the setting, where the soprano and alto move in thirds much of the time (probably to mirror the hymn's simplicity, which was emphasized in early years and in its origin story). The bass is mostly just doing chord roots. The tenor line therefore must fill out the other note of the chord when necessary, and given the low range for the women's voices, it's often at the highest note it could be to allow the women to sing in simple thirds or sixths.
There are only about two places in the whole hymn where the tenor line could potentially go much higher while still filling out the simple chords. (1) First would be on "virgin" and "infant," but the arranger here chose to create a nice voice exchange between the melody and tenor line, which also leads smoothly back to the B-flat of the opening motive. (2) In the first "Sleep in heavenly peace" all the upper voices could be moved up together, but the arranger of this standard version decided (with good reason) to vary the texture a bit and move to an "open voicing" structure with the chords after all the parallel thirds in the soprano and alto for the previous four bars. The tenor does get its highest note there anyway still.
I'm not sure why this is any great mystery. It's a low-pitched hymn because it's meant to be sung quietly (Silent Night) usually late on Christmas Eve in a dark church. The melody and alto are already pitched low, making the tenor squeezed in about as high as it could be with the women's voices in their positions.
Silent Night is a popular song and the melody of the Soprano is quite high, this means it has a wide range (almost 1 and a half octave!) and ends on the lower root note. If so the 3rd has to be in the Tenor, if the setting wants to avoid an voice crossing of the Alt-).
Woman voices in congregations or school classes of today are not used to sing higher than Eb'' of F'', (the same with male voices of men that are not singing in a choir) while male voices (also tenor range) normally don't have problems to sing in this range of your copy.
Even a "normal" tenor singer (with classical voice training) would be able to sing this d of the final chord. But I agree with you: An arrangement for a professional choir could be in D major.
I would even sing up to F4 or even G4 if I sang the melody as a solo piece.
(maybe a major 3rd higher you couldn't have sung the melody)
As you can see: d is in the range of the tenor
Just because tenors can scream top C's (or like to pretend they can, even when they can't) doesn't mean the result is musically appropriate in every song.
For a quiet piece like "Silent Night" is it perfectly reasonable to stay in the bottom half of the range throughout.
The range chart in another answer is misleading. Most "tenors" in amateur choirs (who might really be baritones in any case) are not comfortable above about A4, And writing even an A4 at any dynamic level less than "f" would be asking for trouble.
Top Cs are best kept in captivity in an opera house, and not allowed to run wild in the outside world :)