^A few people have said that a lot of fans care for mostly one group, or have one group with an oshi. Since the time is divided between all groups there are less chances to see your oshi.
For example, if you're a, let's say, Kamikokuryo Moe oshi, you will only see her during ANGERME and maybe one or two shuffle performances, but the time when all the other groups perform you won't see her.
In some years, fans even went out to take a break when a group they didn't care for performed. Or they were really unexcited and just sitting in their seats. Not sure how it is nowadays, said reports were from 2011 or 2012.
That being said, it's easier for groups on their own to fill concert halls than for H!P as a whole.
Personally, I can't completely agree. Zukki had a lot of memorable moments in H!P concerts that I missed in MM concerts.