Probably just comes down to money,
Rugby is niche and will bring very little in, I can't remember the last time I saw someone wearing rugby merch other than England, day to day (at the shops or whatever)
To someone like Castore or Macron getting rugby deals brings more eyes to their brand (I wouldn't have heard of them if they didn't, for example) so they'd be happier to put in a bigger bid, a Nike or an Adidas doesn't care about eyeballs just sales and rugby isn't gonna bring in the bucks
I wonder if some of the more traditional ones (Gilbert, Cotton traders etc.) just can't afford to keep up with the latest technologies etc so don't bother trying to get the high level deals - kit for your local 4ths team is easier to knock out than whatever spray on Lycra the pros are wearing these days, plus all the other stash that'd be required as part of a deal with a club (off field, training, travel, media, warm up, bags etc)