Because it is hell to make software that runs in browser and also looks nice.
It clearly is possible as WagerWorks has shown, but this is much never technology compared to what was available 5 years ago so casinos did not have
the option at that time.
MGS offers flash versions of some of their games though
That's a bit of an understatement. I would describe them as so slow that you would rather gouge your eyes out with a fork.the RTG ones are slow