He lost PN due to tactical errors. He could have started to chase all out at any time, but he didn't. Here it was different. He had already put in numerous attacks, he already wasted energy with that crash and coming back. Wellens made 1 attack, while Evenepoel had made 5 to 10 before. The problem was that the course was not quite selective enough and that someone like Philipsen is ultimately quick enough on short efforts to keep closing the gap and small enough to get decent draft behind Evenepoel. Often it was Philipsen who shut down Evenepoel because he assumed that was going to be his meal ticket.
It's simply not possible to react to every attack, especially when you are the one that has wasted by far the most energy. Paris Nice was an order of magnitude more dimwitted than this.
Wellens was very strong but he had saved his legs for that one attack. Also, iirc, Evenepoel recently came back from altitude, and then missing some punch can be normal the first few weeks.