Why do you think Microsoft built SharePoint on top of ASP.NET Web Forms instead of MVC?

I'll answer my own question because I think I understand the difference (somewhat).

ASP.NET MVC is not absolutely better than Web Froms. The actual reasons might be difficult to know but comparing the differences might shed some light

http://code-desk.com/web-development/pros-and-cons-of-asp-net-web-forms-and-asp-net-mvc.html