Why go Pro when iMac goes faster?: “The new Core i7 iMac is a potent machine — so potent, in fact, that it outpaces a couple of Mac Pro models in benchmarks. It’s also an incredible bargain, offering tremendous power given its price.
Good Question!!!
(Via Macworld.)
aGEEKspot 
Because of the lack of a matte screen option on the iMac. Let’s not get into the matte vs glossy debate. The fact is, some people prefer glossy, other need matte. The fact that many people like glossy does not, in itself, explain away the need for matte screens by a substantial minority, e.g. graphic artists, photographers, and those people who suffer from eye strain exacerbated by the reflections from glossy screens. See http://macmatte.wordpress.com