Because he needed the libertarian vote this time in order to win. Its a very cheap promise that guarantees votes and doesnt alienate anyone that doesnt care. Honestly, the fact that he followed through suprises me.
"But why didnt he need the libertarian vote in 16?"
Because he never intended to win the first time. The first run was a fluke. He wanted to play victim and run the media circuit for profit.
Back then he thought Bitcoin was bad and a scam. This time someone showed him how to make money by creating a scam coin and doing a pump and dump, so now he's pro Bitcoin. So pro he'll get rid of all regulation so pump and dump scams are completely legal.
I bet Trump is still anti Bitcoin. Under the previous administration, the SEC applied the Howey Test to cryptocurrencies and would have identified Trump Coin as a security. Trump is only interested if he can be in charge of it.