The Athletic summed it up
The Phillies were sometimes able to find more favorable spots for Falter because he was the sixth starter. Almost half of his innings (40 1/3) last season came against the Nationals, Marlins and Pirates. Falter had a 3.12 ERA in those situations. He had a 4.53 ERA in his other 43 2/3 innings. Thatâs still respectable enough for a back-of-the-rotation pitcher.
But 5.75: not respectable. And it's actually worse than that. He was decent his first couple of starts and the Phillies even won one of them, but since then, 0-5, 7.01.
He does have a 4.71 FIP. And Phillies have little choice but to use him, hopefully the offense can pick him up a little over the next bunch of starts, and fifth starters are never especially great. But given the flaws in the rest of the rotation so far (even if ERA is also up leaguewide) and the increasing unlikeliness of Painter coming to the rescue, they will need a starting pitcher in July.