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As far as my knowledge goes, stake doesn't accept investors because it totally and completely runs on its own bankroll. There are so many gambling sites which are popular that accepts crypto investments, bitvest, for just an example. Anyone can invest in it with a click.
Too bad that Stake.com won't accept investors. I plan to invest in their website myself but I guess I have to save that money for another website to invest. AFAIK bustabit and bustadice are good website for crypto investments, there was a thread by someone who had invested in both website, I already forgot who because there doesn't seem to be an update on that one.