Pathfinder was created using the D&D 3.5 ruleset*. It’s a much more number-crunchy and fiddly game with a massive set of additional books. D&D 5e is a more simple rule system, and more importantly, it has the feel of older additions of D&D. Many players felt that 4e didn’t feel like D&D. For older players, the 3.5 rule set lost the D&D feel of prior editions, because of the weight of rules.

Pathfinder is much more of a tactical game while 5e lends itself to a more narrative game. This is why you tend to see actual play use 5e as it’s less interesting to hear people argue about bonus stacking, 5-foot moves, and another number crunching**.

It should be noted that there is a 2e Pathfinder in the works, which tends to reduce the complexity a lot while trying retain the 3.5/Pathfinder feel.

*Why Pathfinder was able to, and why they did is a long story, but basically, D&D licensed 3.5 very loosely to get 3rd party publishers. A lot of D&D players didn’t like 4th edition, and the license left 3rd publishers in the cold. So the Pathfinder basically took 3.5 and fix a bunch of things to make Pathfinder.

*There is a subset of players who love this. Personally, I enjoy building Pathfinder characters a lot, but they just are not as fun to play.