what makes a bass pickup only for a bass

The string spacing wouldn't match up from one to the other first off, unless you had a rail style pickup. Plus I assume the bass pickups are designed to specifically handle those frequencies. I'm just rambling off the top of my head though.
EMG's are all rail pickups, and many times people who want to go with active electronics on an 8, 9, 10 or 11 string guitar/bass will go with one. Also, the 5-string bass size (their "40" housing) is a direct fit for a lot of 10-string pedal steel pickups, again for someone who wants to go active with no metalworking.




There's lots more than one kind of nut in the party bowl..... :hello2:

Bass pickups can have a massive amount of windings. This decreases the higher frequencies and gives more low end.