The answer is quite easy: much more injuries.
18/19 Bravo was out, no (big) impact. KDB 1st half was out. Full stop.
End of the season we had several short injuries, nothing big. We won 100% domestic titles and were ridiculously unlucky in CL.
This season:
Sane out -> Sterling takes the full workload on LW and look what happened!
Laporte out, Stones, Zinchenko -> many defensive changes, Dinho saved our arse big time. Even Garcia played 6 full matches already.
Furthermore David, Sergio and Rodri were out for some time.
Pep had to change much more compared to last season(s). And by looking at the players available he had to change the system (tinker), too.
3 at the back came into play to rest Sterling and play without wingers, using 2 offensive wingbacks instead.
Bottom line, we should NOT discuss who is guilty, that would be a trap.
We only have to ask what's best to get the players to the point where they can deliver their normal performance. And score.
Even if the players' shout would be unjustified Pep should be a wise leader and give the team some inches only for motivational reasons.
In summer Pep can present his bill and make the changes he thinks are useful to get more character into the squad
in order to compete with Klopp's bunch.
Otherwise I somehow worry Kev might lose patience and develop other plans, if some things don't change...