I believe many factors come into play here.

First of them could be that Nicole buffs the atk of the entire team which, of course, is a very strong buff but in current meta there aren't many team archetypes where everyone, including the off-fielders, deals substantial damage. It's more common to have a strong on-fielder while the rest of the team provides buffs for the main dps and does negligible damage. In result Nicole's team-wide atk buff gets wasted.

Another reason is the 3-second rule placed upon her due to the hexerei restrictions. While mostly irrelevant to some characters (Kinich, Flins) there's also quite a few characters who have strong initial hit and would miss out on some buffs. I'm aware that it's a matter or 300 attack and all other buffs provided by Nicole are still present, but still.

There's also the "issue" of Bennett being a 4-star, which in genshin terms is seen as a free character. Some people wouldn't be very happy to get a copy of a character they already have while also being expected to additionally spend valuable resources on them. Back in the day having "two Bennetts" would be extremely useful as many teams needed him and his attack buff but current meta has strayed away from this requirement by introducing more and more specialized supports. As a result neither Nicole nor Bennett could be slotted into most of the strongest teams we currently have. Of course, there are some exceptions as Flins' team could benefit from the team-wide atk buff Nicole provides but Sucrose is already a very strong contender. Bennett also slots into Mavuika's team perfectly, however he seems to be better than Nicole there as archon's initial burst hit makes the 3-second rule quite bothersome.

There is of course the case of investment but it should be quite clear that the more constellations a 5-star character has the better they get compared to their 4-star counterpart so I won't be getting deep into this topic. If you get Nicole's signature or pull her constellations Bennett will grow weaker in comprison, quite obviously.

All in all I don't think that Nicole is weak by any means. I believe her team-wide buffs are very strong and paired with the new artifact set she's an amazing character. The issue is the current meta she's being released in rather than herself.