time location in a sentence

If I say "Tomorrow he will be here", the time reference is at the beginning of the sentence, as I would expect:

Míngtiān tā zài zhèli.

But if I say "He will be here in 20 minutes", the time reference goes at the end of the sentence, as in:

Tā zài zhèli èrshí fēnzhōng.

Can anyone explain to me why this is? Thanks!

Ken