Because using large numbers is unwieldy. Imagine trying to buy a bag of chips worth 50 cents. In whole bitcoins that would be .0005 BTC. To speak the words "point zero zero zero five bitcoins" is awkward and can cause confusion over decimal places, especially if BTC goes up another order of 10 or more.

The psychological value of switching to milliBitcoins is more than worth it, especially if the endgoal is to someday use BTC as a viable alternative to fiat currencies.