Yes, you can certainly get a UTI from a diaper. I think it depends on your immune system more than anything else. As others have said, urine exposed to air is a bacterial breeding ground. There seems to be a wider variance in people's immunological systems than is often recognized.
It has never happened to me, but my immune system would be well characterized as "aggressive." Take my experience with a grain of salt. It causes more problems than it solves, so far as I can tell, but the upshot is that I almost never get sick with anything. I've only had the flu once, ever. That probably helps with diaper wearing and bacteria. The downside is that I need to carry EpiPens with me everywhere, because my immune system might well get carried away and go nuts over, for example, parsnips. My immunologist calls me one of his "interesting" clients.
Having said that, UTI's do not seem to be a major issue here on ADISC. I have not seen many threads about it.