There's a lot of pressure that goes along with taking monthly donations. I felt persistent guilt when I wasn't working which means I was pushing myself to work 12+ hours a day. Many developers do the opposite, they know they have a monthly guaranteed payment coming so they lax on work. It also inspires developers to never complete a project. When I announced Avalon would be complete, 30% of my Patrons dropped support. It still took me four or five months to complete it too. There are some developers that stretch out their releases for months just to keep a project in production.
It's not that it's inherently a bad system but it certainly has its pros and cons. Selling my product when I complete milestones feels more natural. I can work at my own pace and I get rewarded for accomplishments which feels more gratifying. It allows people to pay for a tangible product instead of simply having faith in the artist and donating based on the artist's previous work and their promises.
The major reason though is probably that I have a difficult time with jealousy. I have a bad habit of comparing my numbers with other artists in the community and I get extremely depressed when I'm significantly less popular than others. It's something I'm trying to work on but currently my sin is envy.