I'm an ex-Quicken Loans employee, and my best guess is that Dan enjoys Comic Sans so much because of the reaction it gets. The billionaire equivalent of trolling.

To that end, while working on the web team we added a Konami code to the QL website that would change the entire site to Comic Sans. I just checked and sadly that Easter Egg got lost in a redesign.

Yahoo and Comic Sans. Together, in relevance, stuck in the 20th century.