A week or so ago, a blog post was posted in this Community calling out Mullvad for using GMail as their email provider. Wasn’t the greatest blog post in the world and didn’t approach Mullvad for comment or explanation. Anyway, looks like Mullvad heard about it and responded.
I think they probably did it at first as its quick and easy to set up. And they did strongly recommend anyone mailing them encrypted the emails. I would also assume it was always the plan to self host them but it was the least important part of the whole system so they left it until last to address.