Why can't I create a fork of BTC that has no difficulty changes so I can premine the rest of the coins?
Depending on what you meant to ask, the answer is one of the following:


If you run a Bitcoin-qt fork which is incompatible with the rest of the network, you're running a separate currency which has no value (because no one else accepts it).
If you ignore blocks and forge historic timestamps to create a branch with more blocks of low difficulty, it will not be chosen over the main branch because the choice is made based on total difficulty, not number of blocks.