For website monitoring, a big part of the reason that people buy is trust, not necessarily ease of signing up. When you're running a commodity business at scale, you'll also find that more and more users will sign up for free accounts repeatedly over time instead of paying you money, which is yet another reason to require credit cards for free trials.

When you're doing most things yourself, like me, the goal is to get a specific type of customer (not every customer is a good customer for you, some types are a bigger drain on support resources than others), and the easiest way I've found to get those is to require a credit card for the trial. That might mean that I have less users overall, but I end up with the ones that I want.