the reasons are multiple.here are the quick few i can think of off the top of my head inside 5 mins:
1.the stock mechanical fan clutches are failure prone.
2.the stock mechanical fan,when locked up robs hp.rather than loose your momentum going up the hill,due to hp going to cool your engine,the hp is left at the wheels to take you up (electric fans,even two,on high,cannot be felt at all.)
3.the stock mechanical fan is heavy and therefore puts more strain on the water pump.
4.the stock mechanical fan "can" and does,have a tendency to fly off and as a result,a many idi owner have lost radiators this way.
5.the stock mechanical fan always spins even when unlocked,which means it's always using some of your fuel to spin (nothing can operate for free.it requires an energy source)
6.many stock ford e-fans (and other brands) have been known to last an extremely long time before replacement is required.
7.frees up engine bay space,making for a cleaner look,along with ease,and safety when working on the engine when running.
8.idle,or just off idle airflow can be brought up far past the cfm rating of the stock mechanical fan on the fly.this is useful for those who work/drive off road/slow zones,where more cooling airflow may be desired,such as when using A/C.much cooler air as a result.
9.ease and cost of replacement.a lot easier to swap out an e-fan motor (just a min or two) and the price is well under $50 used.
10.increased throttle response.
11.quieter operation.
