Originally Posted by uniquemkt

Because:

a. Mining hardware is expensive. The faster it is, the more expensive.
b. Faster hardware takes longer to get, and
c. By the time you get it, it's already outdated.

ASIC devices changed the playing field, GPUs are as useless as CPUs became. The big name ASIC vendors are actually shipping the pre-orders from a year ago, and at the rate these things are hitting the network, the insane moneymaker you ordered last year and juse received is making $6/month worth of BTC, and getting lower every day.

Take whatever mining hardware you want, get it's hash rate, and plug it in here: http://www.bitcoinx.com/profit/
Then look at the difficulty chart: http://bitcoindifficulty.com/