Legal reasons will be the most likely cause. It'll certainly be the biggest reason in the US (and here in Australia, FWIW) where the owner companies aren't allowed to have anything to do with online
gambling. And even in countries where they are permitted there's still the possibility of legal action if they aren't able to police who does and doesn't play on their sites.
Other than that, the skills you need to run a successful internet
poker site or online casino are
very different to the ones you need to run a successful brick and mortar one. I expect even quite a few of the casinos that could legally provide the service in their jurisdiction are probably tossing it in the too hard basket given that they'd have to hire big teams of programmers and IT support operators that the don't currently have to bother with.
Lastly, online gambling is already a crowded marketplace. Using the name of a brick and mortar casino that's built up some reputation and brand
equity would help to stand out in that marketplace, but to make any real difference it'd have to be a
very strong brand that's recognised globally. The only ones I can think of that would qualify are the Vegas-based ones that are firmly disqualified for legal reasons - the likes of the Bellagio, Wynn, Hard Rock, Mirage, Horseshoe, etc.
Would Star City poker room really stand out from the bajillion other online poker rooms, for example?