In ASP.NET Web Forms, passing information between different pages was never especially easy without the use of session. Due to the postback-centric model, information was available on the server as part of an event, but often in the wrong page for displaying a result, making passing information between pages necessary.
This tended to lead to an overuse of session, populating “current” variables in session intended to indicate what the current object being interacted with was. This overuse in turn made applications very state-dependent and much harder to determine expected behaviour (“Is this variable populated?” “Do I have the current order ID yet?”).
MVC is structured around the idea that your website is a view into a logical model of information. It encourages having stateless operations through the use of simple controllers responding to actions with key information passed as part of the HTTP request.
Because of these properties, session is no longer required to perform basic tasks in MVC, and becomes poor fit where it has seemed a perfectly valid choice before.
Fundamentally, session pollutes HTTP. It makes requests (often containing their own state) dependent on the internal state of the receiving server. This is why it’s seen as something of an evil (although often a practical and necessary one).