Simplicity increases the accessibility of my designs. If my games are easy to teach, learn, and play, they're more likely to appeal to many different kinds of players.
Of course, I'm not just designing for the masses. I myself dislike learning complex rules, so I try to design games that I would enjoy learning.
Even more importantly, I hate teaching complex rules. When faced with the prospect of teaching even a relatively simple game like RAMbots, a wave of exhaustion washes over me. In contrast, it's a pure joy to teach a really simple game like Uptown.
I can't overstate the impact that this contrast has had on my design style. Every time I consider adding or changing a rule, I think about how hard it will be to teach. For the foreseeable future, I doubt I'll be designing any game that's much more complex than Uptown.