WebForms' selling point is that it is easy to bootstrap, but its limitations quickly become a liability in an enterprise setting, where small apps rarely stay small. Once you decide you need to extend your app and/or integrate with newer non-MS web technologies, you are backed into a corner.
WebForms is something that really ought to have been deprecated, but people are still clinging onto it because it's so easy to use and there are too many legacy apps that still depend on it.
I suspect it's the kind of Stockholm Syndrome that have kept Mongo users faithful over the years. At the back of your head you know you're going to have to pay down the technical debt later, but you can't resist because it's so easy to get started and to see results.
(though to be fair, Mongo 3.4 onwards has overcome some of the serious flaws of earlier versions)
If you have a massive complex webapp then you should use MVC, or look at a separate frontend JS talking to a backend API, but WebForms serves its niche very well.