Question

why do we prefer to use coils to decrease current over capacitances in case of ac?

Answer 1

Both are used. Watch this lecture beginning at 37:00 where he demonstrates a variable inductance coil, but then goes on to discuss the use of variable capacitance:

http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-25-driven-lrc-circuits-and-resonance/

Answer 2

A coil, has a very good property in this case that I opposes the flow of current and stores the energy dissipated in form of magnetic field, around it.
when a electron go through the coil, it carries electric as well as magnetic field , when it passes the first turn of coil, it will make it a magnet, but when it comes to the another turn the first coil magnetic force won't allow it to pass through it, so the electron need to apply more force and energy to overcome this so, it ask from the source for that energy, hence after the electron has left the coil, the energy of electron has stored in form of magnetic field in coil, hence slowing down the speed of electron (as less energy pressure will impact less). And, we know flow of electron with respect to time is current, as time increase due to decrease in speed, hence Decrease in current is definate.
I hope you understand......