Why does the glideslope use UHF when the localiser uses VHF band?
The higher the frequency the smaller the transmitter, so fitting the G/S on a small tower near the touchdown zone becomes practical. Compared to the large LOC array at the other end of the runway.


  [Of] the advantages of using UHF frequencies for glideslope operation is only a small antenna and ground plane is required. As a rule of thumb, the higher the frequency, the smaller the antenna can to be (avionicslist.com).


Here is a table for the localizer and glide slope frequency pairing (fcc.gov).

As for why not UHF for the LOC, my understanding is the range. A pilot can receive the LOC farther out than the G/S (8 miles more).