From what I hear from my friends who use Uber, 1 reason they love it is because all the charges are through the app and there is no "obligation" to tip, so it's 1 less decision them have to make, i.e. calculate how much to tip, keep wondering if you tipped the driver too much, or too little, what's a "fair" tip for the ride etc etc.
I will never tip with zero guilt.
If you think this is not a real obligation or doesn't have real consequences, then you have no experience with tipping.
I suspect you're in the minority. Most people (in the US, anyway), when presented with an explicit option to tip, will do so out of a sense of obligation, or out of a desire not to be seen as a cheapskate -- even if the decision whether or not to tip is anonymous! Unfortunately that's how ingrained tipping culture is here.
That's an opinion, not a point. There's little data available to support the assertion, other than that Lyft has been, and still is, a smaller organisation and less less popular globally, and hasn't been growing as fast as Uber. Ultimately you can't compare the success of Lyft-with-tipping to Lyft-sans-tipping since one doesn't exist.
> It's not hard for people to use--which was what the GP was speculating about
I agree with others (mostly non-US residents, I expect) insofar as I don't think it's hard to physically tap an extra button on my phone, but it's additional cognitive bother that didn't exist yesterday.
It de-differentiates Uber from its competitors (this doesn't really matter to me, but it does make the background to the decision more interesting).