He got it about 5 years ago. His Twitter feed was full of btc. And him and Saylor went back and forth a couple times. Tesla was running it's own node and was gonna start accepting btc for cars. It's all still there. Can search it. Don't know what happened.
Discussion
I think the simplest explanation is that people are just people. So is Elon Musk. I believe he was (and still is) heavily influenced socially by his peers in silicon valley, which tends to favor the crypto pluralistic ethos over the bitcoin-only ethos.