Romney's health care bill is perceived as virtually the same thing as the widely detested Obamacare because it is virtually the same thing as the widely detested Obamacare.
Both Romneycare and Obamacare include the individual mandate. Both Romneycare and Obamacare include sharp increases in Medicaid. Both Romneycare and Obamacare include guaranteed issue and community rating (like requiring fire insurers to insure homes that have already caught fire, and at the same standard rates as for everyone else). Both Romneycare and Obamacare include welfare subsidies for the purchase of health insurance well into the middle class. And both Romneycare and Obamacare include the latent government power for price controls on health insurance and rationing of health care.
The problem is not that President Obama is a raging socialist/leftist/fascist trying to force a health care mandate on hapless stalwart Republicans. No, the problem is that the Republican party leadership has become so immoderate that President Obama’s obviously moderate Republican idea is no longer acceptable to them even though the contents of his legislation are, mostly, accepted by rank and file Republicans.