Why are lower VHF frequencies used for towers?
There's no specific reason why the lower frequencies are used for towers, but it's not a coincidence that they are grouped together (mostly, there are tower frequencies that are not at the lower end.)
Spectrum management is a complex effort to make maximum use of the fixed amount of frequencies available for use. In the US this falls under the FAA Spectrum Engineering Group AJW-1C.
One method of ensuring reliable safe communication is the intersite analysis. Basically, frequencies have to be used by multiple ground stations based on the airspace volume that the frequency needs to cover. This airspace is referred to as a frequency protected service volume (FPSV).
The goal is that an aircraft in the FPSV will only be able to communicate with the intended ground station. Since towers have a relatively small FPSV, typically 5,000' high (AGL) &amp; 15 nmi radius, the tower frequencies can be reused closer together than a high altitude sector frequency that may cover a 150 mi radius at 45000 feet. A frequency that was used for both high altitude and a tower would be more difficult to de-conflict.
There are also other considerations that can affect the assignment of frequencies. It's all spelled out in the 6050.32B - Spectrum Management Regulations and Procedures Manual. It's rather lengthy at 475 pages. The details of VHF/UHF Comm frequencies is in Appendix 2.