Because an EMP is basically a big Magnetic Pulse fueled by atomic sized reactions which was caused, in most cases, by fusion. EMPs vary in strength depending on the source, the sun sends out an EMP which sometimes interfere with technology but a nuclear blast would not have an EMP of that range. The Crucible was a powerful source intended to disable thousands of Reapers at once. There were countless Reapers that outnumbered the galaxy and made conventional warfare with them impossible. By the time they killed off Reaper in another planet, a planet somewhere else falls. SO even if the galaxy made an EMP big enough, they did not have enough numbers to take on the entire Reaper armada. Hell we don't even know if only a part of the Reapers came to the galaxy while others remained in the dark. Or even if they are from this Galaxy.