i don't think Berryz is doing anything different than any of the other groups. if they had a cohesive look (meaning less members that stand out in odd ways) and music that people can more easily respond to, they'd be doing pretty well for themselves.
but they don't have control over either of those things, so they do their best with what they have. same with ℃-ute and Momusu and S/mileage and Juice=Juice.. doing the best with what they have, and it just happens that what ℃-ute and Momusu have (too soon to tell with J=J imo) appeals to a larger crowd. i'd say the only individual making a difference for Momusu is Sayu, and Berryz has Momoko in that field. for Momusu it was mostly the refresh of 9 new (and appealing) members that made the difference, and Berryz can't do that.
℃-ute certainly isn't putting themselves out there personality-wise, they had many years when they were less popular than Berryz and they weren't doing anything special about it except trying to improve their skill, which all groups do. Berryz just can't show that off as much because they have more members
and more diversity in the skills of their members, so while some have improved at dancing, some have improved at singing, some have improved at expressions, and some have improved at talking, there's always someone who is not good in those areas that drags the overall level of the group down. and then they still have the awkwardness due to their lack of cohesive look to contend with.
so basically it's more of an uphill battle for Berryz than it is for other groups, except maybe S/mileage (who has similar problems). and so they may be trying just as hard as other groups, but they won't see as much results for it unless they get a really freaking good song. if anything, what they need to do is to be trying harder than everybody... they have to be willing/active about doing things when they get a TV appearance that ℃-ute wouldn't do, and milk every opportunity. and maybe revolt when Tsunku gives them a song they don't think will help them in this journey lol.
but they may not necessarily do that or be able to do that, and i don't blame them. it's been 10 years since their debut, at this point some of them would probably rather graduate than face the difficulty of trying to catch up in popularity. or, continue as they have now up to now.. keep trekking along even when they aren't doing so well, and maybe they'll eventually get a look and a sound that will appeal to a wider audience.
i still think the group needs to be split in two (unless either they or Tsunku or UF have a surge of inspiration for getting the group out there as it is now). look at this perfection in the center, those 3 would make a very appealing unit.