That's Why Kim Deal Left
It would seem to me that Kim Deal left Pixies because the new songs weren't that good.
If someone else has an alternative theory, I would be glad to hear it. When it isn't working in a band, and when people don't want to hear your new songs, you either leave or soldier on. Deal decided to leave at exactly the moment where releasing new music became an issue. What's refreshing is that this probably wasn't about drugs or money which is what it's normally about.