In a way the Cubs can take some credit. First, them being very good and still on the Brewers heals in spite of the run they went on forced MKE to keep their foot on their gas. They had that 18 in 17 type stretch and then another couple weeks after where they had to try. This likely contributed to several pitcher injuries towards the end of the year, along with extra use on the good guys who did stay healthy. In addition, stretching them to 5 in the NLCS put them behind the 8 ball going into the Dodgers who of course didn't need any help.
That said, pitching was not really an issue. It really just came down to the same Achilles they've had the last 7ish years, not enough O vs elite playoff pitching. Maybe a few off days in Aug/Sept help a bit, but its tough to say the hitters could be all that burned out or tired with it being over in the last 1-2 weeks plus then having like 6 days off before the playoffs. Hitters just didn't hit after game 2 vs the Cubs and I'm not sure how much could really be chalked up to being tired considering the 2-3 weeks prior were cruise control and off.