Why would a flight take a detour on clean weather?
There are lots of reasons flights may not take a direct route, some are but not limited to: 


It's just the route ATC assigned (for whatever reason they see fit)
There is an active military operations area or live firing area they are avoiding, these are not always "active" and may be avoided only sometimes.
Some kind of natural event on the ground, wildfire, volcanic eruption, etc. 
There is a TFR they can't get through these are typically not hard bans on overflight but can be. 
If it's an international flight they may not be allowed to overfly the airspace. 
For smaller aircraft it may be a terrain avoidance issue (generally this is for general aviation sized planes).