Because FUN token is their native utility token. They want users to purchase FUN to boost the price in exchange for the benefits in the casino. FUN utility will be useless if they let other currency use to be staked since no one will by and hold FUN token since everybody preferred blue coins to hold long term.
Actually, FUN token is the only casino token that I knew that gives real utility to their token since they provide different reward that involves the casino for token holders unlike mediocre casino tokens that only use as rakeback rewards or stake like DeFi that doesn’t gives much benefits for holders.
Looking at the FUN token chart, I'd say almost no one buys or holds it anymore. It would seem that the integration of this token on the freebitcoin site should have created a huge demand. However, as we can see, it isn't happening even though half of the FUN token emission is locked on the freebitcoin site.
They messed up with this and their FUN token is affected to their past issues happened. I don't know if there's still a chance for FUN tokens to recover and its seems many people is avoiding to acquire this token.
But let see since right now it seems they are doing good and if they get back in their normal operation let see if the interest of people who's holding or buying FUN tokens before will acquire it again. For now they need to work a lot since lots of people lost their interest to gamble in their casino.
For now I am one of those people avoid to get some FUN token since I don't want to take a risk for their token at current situation.
What happened with freebitcoin was a shocker and many of their customers have really not find way out of it, at least not for now, and freebitcoin would need to put in a lot of hardwork to ensure they start regaining their customers trust and confidence as it were.
From what have seen recently their operation is gradually improving and so with time users will feel convinced to get back to holding their FUN token. And another is, if they can reinstate their signature campaign on the forum too, it would really go a long way to change the narrative and impression of the community too.