They are a lot cheaper. Maybe they didn’t want to pay for an expensive .com domain. The prices can be nuts, but usually only for short named ones.
What? .com is generally around 10$/year and .me around 20$… In the grand scheme of things the cost of even the most expensive TLDs are inconsequential to a large operation like this.
Now if you’re talking about buying back a domain from someone, sure. But that’s a one time thing, and proton surely could afford tens of thousands of dollars if they wanted to.