Why do some airfields not have manned ATC towers?
One of the biggest reasons to not man a tower is (not unsurprisingly) costs. 

Many remote airports only have a few movements per day, and a permanent staff in the tower makes it even more difficult keep the operation economically viable.

For this reason, remotely staffed towers are being developed now, and for instance Örnsköldsvik airport (ESNO) in Sweden has just received approval from the Swedish Transport Agency to operate with a remote tower:


  The system’s sensors are installed at Ornskoldsvik Airport but traffic
  is remotely controlled from Sundsvall, Sweden.