Why would anyone willingly choose 1FA, when you have the opportunity to go for 2FA in more than one way. Who are these unicorns, that are opposed to 2FA?
From a service point of view, MFA can be deceptively easy to implement. Over the past year or two, it's become all the rage due to aggressive and effective credential-stuffing campaigns.
Unfortunately, it's also difficult to get this right. You can end up with users permanently locked out, you can introduce SSPR abuse, you can increase demand for SIM swapping and email account takeovers.
It's better than what came before, and it's part of everyone's Defense in Depth strategy, but it's not a silver bullet.