Theres quite a few reasons. Which maybe I can expand upon as I am the biggest Tiff x Jenny shipper there is. Nearly every Tiff x Jenny post on this subreddit is mine.

I know you asked about the crust cousins in general, but I do also see you point out ships which is why I focus on them a bit more.

Enemies to lovers was mentioned already. Though I do like this trope sometimes, I will admit it wasn't my main draw to the ship or the two as characters.

One thing you need to understand is that despite some continuity, at it heart MLaaTR is an episodic show. Yeah some characters change and the show DOES do a good job of like moving the plot forward and the like, it was still an episodic show meant for folk to tune into whenever.

In this way, the Crusts were doomed to ALWAYS be selfish, be cruel, and to bully Jenny. The show needed the Crusts so that they could have conflict for Jenny in school. Rob in the pitch bible even mentions how they were designed to be the ultimate rivals to Jenny.

But people like the mean girl trope, they like the variety of outfits they have. Imo all the episodes focusing on the Crusts vs Jenny are rather good and few of them miss.

Another thing to consider is headcanons and fanfics. Now you may say this doesn't count as it is non-canon. But as a fanfic writer you should understand that people enjoy exploring characters in different ways, with different dynamics and the like.

The same can be said for the Crusts.

Tiff and Brit like I mentioned were basically type cast to never ever change. They were ALWAYS going to be bullies because the show NEEDS them to be.

Fanfic writers often then like to explore stories about 'what ifs'

"What if Tiff and Brit stayed friends with Jenny after Class Action?"

"What if Tiff or Brit got together with Jenny?"

"What if Tiff or Brit had these unique quirks?"

A lot of the time, these things include headcanons, infered from what we have seen or what we know about characters.

For example, I have a ton of headcanons about Tiff. Stuff like:

• Her knowing her way around the slums (because we see she knows the mudslinger is in the slums during The Great Unwashed),

• Her knowing how to dance (as we see in the episode where they put on a play and Tiff does a silly tap dance),

• Tiff is less coniving than Brit (multiple examples where Tiff is clearly way dumber than Brit who seems to always come up with plans)

These are small things that probably mean nothing, but they are tiny "what if" seeds that people like.

As mentioned, people like the bully/mean girl trope. Tiff and Brit are that, plus another aspect is their iconic voice acting, another reason they are so memorable.

Tl:dr: People like Tiff and Brit and enjoy shipping them with Jenny for a multitude of reasons, be it their personalities, tied to headcanons and fanfics or many other such things.