Well... 1st of all, the spectrum is a very scarce and expensive resource and its far more than just interference as a factor to contribute to the decision-making process when deciding what portion or what band to allocate to a specific technology.

There are all sort of economics and sometimes politics when deciding on a frequency band. I'll give you some technical overview on how to decide a suitable band, and thats my opinion and understanding off-course!!

High frequency band = major propagation losses; free space and absorption

Usually used in line-of-sight environments, microwave links is a good example. Although high frequency means multiple paths for me in a urban environments; but its not practical due to the expensive propagation losses.

Low frequency band = Big antennas, sometimes very bid antennas !!, fewer propagation losses but not practical for handheld devices, radio and analog TV are good examples.

Now in between these categories there are good frequencies that have reasonable propagation losses, suitable multi-path characteristics (No line of sight required), integrated antenna design and low complexity..

Now, you need to understand that spectrum managers and regulators like ITU cant just ground one band and use another one!! there is already a massive investment in previous technologies and they can just chose the best band!!! they have to compromise and find a compromise solution. Otherwise, No one would consider investment in this technology (economical). 

From this, they start looking at neighboring technologies that co-exist on the same spectrum (sometimes) or that are adjacent with potential out-of-band emissions.

You cant just think of interference because, in my opinion, could be managed with stringent technical specifications and proper planning.

3.5GHZ band is popular for wimax because in order to avoid interference in the network and to achieve better RAN KPI values, we could move to a 4‐frequency plan from a 3‐Frequency plan. After the execution of 4‐Frequancy plan, we could be able to enhance our Effective Isotropic Radiated Power (EIRP) to the maximum level (384 dBm); in this way it could provide interference free propagation of signal, provide better quality of service and this band reduce/eliminate interference due this execution of 4-Frequency plan.