“We felt it was time for a change. This might sound weird, but while we got very good at making ‘Zachtronics games’ over the last twelve years, it was hard for us to make anything else. We were fortunate enough to carve out a special niche, and I’m thankful that we’ve been able to occupy it and survive in it, but it also kept us locked into doing something we didn’t feel like doing forever.”
So they basically got tired of making more Zachlikes, which is more than understandable at this point. Amazing games, but I think they can safely say their job is done after they've basically kickstarted a genre. Rather than being sad seeing Zachtronics go, I'm sort of anticipating to hear more about some of those smaller "weird" projects Zach alludes to in the article. Would be much more tragic to see a designer of that caliber left feeling stuck making games in a similar vein for longer than they feel is necessary.