Though in Roblox's case, there's two additional factors helping the success of games on its platform besides being free to play
- Roblox has become the de-facto portal from which lots of people play games by default, especially on mobile devices like tablets where discovery for other games (that aren't P2W and can spam ads) is very poor
- Multiplayer games are exceptionally easy to develop on Roblox. (With a standalone game you have to grind on an engine for years like what I'm doing. I'm developing thousands of LOC per weekend with a multi-agent setup and there is just so much necessary complexity that launching an alpha build will take months.)
Minecraft (Mojang/Microsoft) have also made it clear that with them moving from OpenGL to Vulcan they're maintaining the ability for Minecraft to run on Mac as well as Windows/Linux, which is fantastic.
My bet is that the real different lies in mobile devices - iPads/tablets and phones are something that kids have more access to than laptops or desktops, and lots of people don't bother with parental controls.
Roblox is getting pretty bad though with age verification, and they will NOT help recover a lost/hacked account no matter how much money was spent on it. 5 year account, likely thousands of dollars in robux spent on it, kid clicked a bad link and it's gone. Roblox just responds they can't help, even though the original email address was verified, and used 2 factor auth through its history. They used to be helpful, they don't care anymore.