It's mainly the fact that bustabit is the preferred choice of game for megawhales like whatevvs, FREE-JBAY, KLITZ, etc. Probably due to them being around since 2014 and always processing large withdrawals instantly. There is no crash game in the world that has a higher trust level than bustabit. Just a few days ago, the whale PredictableLegacy lost 220 BTC within a few games. I mean, it could have gone the other way too... I guess luck/variance is also a pretty major factor. But yeah, everything seems to be going very well for the two sites, and I hope it continues.
I actually don't think whales had much to do with it. Bustabit in the original days was (intentionally!) very unappealing to whales. For a significant time, while it underwent a significant amount of growth with a max bet under $100 USD. So i think the whales came later, after it was popular and had a big history of being trustworthy.
It's hard to know for sure, but I suspect one of the biggest factors was actually the withdrawal system.
As you mentioned, bustabit has consistently processed huge withdrawals instantly without bullshit. Pretty much everyone else screws this up. It blows my mind that other casinos will make people wait a day or more days to get their winnings, so they can "investigate" or what ever. You often get someone who's super happy about having won money (and thus had a great experience) and then stress them out while they're anxiously awaiting their withdrawal. I realize there's a security trade-off to be made, but I'm pretty sure that instantly processing huge withdrawals and wins is the right way to go.
I also think busta* is a pretty neat size, where it's big and professionally run enough so you know it's not going anywhere. But small enough you can still easily talk to the owner, get good support etc.
