This is just a pro forma meeting between two heads of state with diplomatic relations. It won't be substantive, and it won't effect the political capital of either Trump or Francis. It's a one day story.

On the other hand, had Francis refused to meet Trump, it would become a big, lingering story ("Pope's feud with Trump continues!") It would make Trump look sympathetic ("Hey, I reached out to the pope, but he snubbed me!") while making Francis seem petty, and it would provide more fuel for Francis' critics.

Meeting with Trump will be like ripping a bandaid off. It has to be done, so better to get it over with quickly and then go on about your business.