The first is that self-driving is super difficult, they don't really have the expertise, and a lot of their progress seems to have come from being able disregard proper safety procedures. So to move forward with it they'd need to fess up to investors that it'll take much longer than anticipated and it'll be much more expensive. That'll damage the company value significantly, and it's not really relevant to what the core of the business is doing right now. I actually find it fascinating - Uber has built an app that disrupts the traditional taxi marketplace. Separate to that they've got a division working on a produce to disrupt Uber's current market place.
The second problem is that if they choose not to do autonomous driving their entire business proposition needs re-establishing. Can they actually make money doing what they're currently doing? Or are they doomed to sink huge venture capital sums into acquiring market shares, only to fail to reach a dominant enough position to actually raise prices and make bank. And part two to that question: Can they achieve that profitability and a good enough return to be worthwhile for investors before someone who does succeed in disrupting the taxi business with self-driving cars.