Plus the fact that there is a really rich ecosystem around Bootstrap with third party templates that are VERY well written and documented. I am not too proud to say that I routinely go to wrapbootstrap.com to purchase a template for ~$20 and use it as the basis for my web apps. Users love it, and I save having to hire or contract a dedicated front end designer.
Also, the whole "every app looks the same" opinion is only true if you're a developer or a serial startup beta tester. Most actual users who'll pay for your product don't see that many SaaS apps, so they don't realise so many apps use Bootstap. Their opinion counts for a lot more than a typical HN reader who might see it several times a day (obviously this isn't true if your market is HN readers).