As has already been said, people here aren't buying more primarily because we've been around for enough buying opportunities to fill our appetite. Money is a finite resource and any investment has it's logical limits - regardless of what the company is, there is a prudent level of exposure that one should never cross.
For those specific nay-sayers that say "If you are so optimistic, why aren't you buying more," I pose this question: If you are so sure it's going to fail, why aren't you selling everything you own and shorting with every dime you have?