I think it kinda mimics schedule for the teams.
We are at tail end of EPL season which is a tight one - for reference Arsenal played April 3, 6, 9 ( CL leg 1 ) , 14 and 17 ( leg 2 )
Bayern played Mar 30, 6, 9 ( leg 1 ), 13 and 17 ( leg 2 ) having already conceded Bundesliga
Arsenal locked in 3 way battle for EPL.
MC same thing. March 31 til yesterday- MC played in 6 matches. Real Madrid, in same time frame, played in 4.
Yet teams like Arsenal and City, locked in battle for league title dont have the luxury of dropping points in domestic league while still trying to be competitive in CL.
Real Madrid 8 pts clear of 2nd place in La Liga. So they can afford to rest some starters in domestic ( mid CL ) matches whereas City cant.
I just think its a matter of where a team is in their respective league that lends to issues late in CL play ( Q, Semis )