I think it's mainly because if you're only mining for 90 days, there's no ROI.  

Using the stats from CoinWarz:
10 Ghash = 0.00048099 BTC/day
0.00048099 * 90 = 0.0432891

Net loss of 0.0052109 BTC - and that's assuming the difficulty stays the same for the next 3 months.