It's not. It's worked fairly well, and some of the shortfalls are the result of deliberate sabotage. For example, the program was supposed to expand Medicaid, our health insurance system for the poor, to actually cover everyone who is below the federal poverty line. But many conservative States chose not to expand Medicaid which leaves many of their citizens without coverage.
The opposition to Obamacare is almost entirely unrelated to its merits. After all, Obamacare is based on a proposal which originated in the 90s with a right-wing think-tank and then was put into place by a republican governor in Massachusetts. Initially, Republican politicians opposed it for purely partisan reasons. Over the years, their opposition to Obamacare became part of the Republican identity. To be a good Republican you had to oppose Obamacare.