Because most of the time when trickle down concepts are proposed (I say most because Romney is at least smart enough to know that he can't just slash taxes and leave it at that with things being the way that they are right now), the candidate trying them doesn't bother trying to make them revenue neutral. They just cut taxes.
We spend a lot on Medicare/Medicaid and defense, and no one who cares for their political career is going to tackle those sectors anytime soon, so how does decreasing taxes for the rich mean the middle and lower classes aren't going to be picking up the slack at the risk of increasing our debt at an even faster rate?
Reagan himself didn't even bother, since the economic base was good enough at the time that the massive debt accumulation of his administration (both from cutting taxes and from the massive defense buildup that occurred concurrently with it) wasn't really a problem in and of itself (well... it was in the sense that massive debt accumulation is always bad and it arguably established a dangerous precedent, but no one really cared at the time because it was benign then).
Edited by Gilda, 01 August 2012 - 05:17 PM.