Bitcoin is more convenient to use unlike the other cryptocurrency. Bitcoins is now considered valuable as real money. Its more easy to use because you dont need to exchange it to dollars just to pay bills and cash out. Bitcoins can also use for any investment. Alsl the low transactions fee on bitcoin transaction

Because it was the first one and the original, I guess. Others are just a copy of the blockchain. Maybe there is only very few that has improved version and they could emerge as good competitor of bitcoin someday.