Hi lepi. Another long time not see.
It's about the density and the sequence of patterns. Muzu is relatively hard as you said(probably already Oni level?) because, even though patterns like 1/4 are monotonically, their sequence is kinda demanding with all these duplets and triplets bounded together with 1/2 (e.g. 00:46:905 - ). In that respect, the density is another factor which makes the Muzukashii harder than an usual one. Lower players have baerly time to rest for a moment due to the continuous mapping in the difficulty, which is why deleting some notes would be a possible suggestion by some of us.
That's the reason. Uuh... kinda stricter compared to the past, or? Well, nowadays we are really caution with the difficulty of a map(and spreads; another topic) since we want to prevent having in the future monster Kantan, Futsuu and Muzukashii difficulties(as osu kinda provides already). So one argument we often give, under consideration of the song of course, is: If you guys want hard difficulties, you can make an additional Oni instead of making the lower diffs harder.
In your case, you could either have some breaks in the Muzu or just rename Muzu to Oni and make a new Muzu if it doesn't bother you too much. Don't know if you can agree with that. A lot of stuff happened which lead to this.