Because you'll get the same issue as with the homeless, people from other states will come over to mooch on the better services. This has to be handled at a national level in order for it to succeed.
Putting that aside, the theoretical prospect that states would have to accommodate out of state moochers doesn’t seem to be the actual reason that the California and Vermont universal healthcare efforts failed. It seems like those programs failed because voters don’t want to pay for it.
Considering many large companies self-insure their employees' healthcare, it seems unlikely that simply expanding the scope of who is covered is going to make the whole plan work.