We’re used to talking about how unified Republicans are — they approved President Trump’s Cabinet appointments almost unanimously, for example. But the House’s health care bill, which was unveiled on Monday, mostly seemed to divide them. Members of highly conservative groups in the House, such as the Freedom Caucus and the Republican Study Committee, have already bashed the draft legislation for maintaining too many of the policies and principles of Obamacare and for adopting a tax credit that they say is a “new entitlement.” However, conservative health policy wonks, whose views could be in sync with more mainline factions of the GOP, have critiqued the bill from another direction, saying it would considerably increase the number of uninsured people. Even the White House’s initial reaction was tepid, although Trump called the bill “wonderful” in a tweet on Tuesday morning. I’m not sure how I’d go about forecasting the probability of its passage, but it didn’t have an auspicious debut.

Health care is a notoriously difficult issue to tackle — most presidents have failed in their efforts to make wholesale reforms, or paid a substantial political price for attempting to do so (or both). But it’s worth considering why this is the case. Part of the answer is that in contrast to something like a Cabinet nomination, which is a simple yes-or-no question, health care is much more multifaceted, allowing for a myriad of possible legislative solutions.

Authors of the Affordable Care Act, for example, have sometimes referred to the “three-legged stool,” where the legs can be thought of as affordability (the government helps people pay for their insurance if they can’t afford it on their own), access (people can get health care even if they have a pre-existing condition) and cost control (there’s some mandate or incentive to ensure that healthy people sign up for insurance, avoiding the death spiral that can result when the proportion of sick people in the insurance pool is too high). If one leg of the stool wobbles, the whole thing might collapse; removing the requirement that insurers cover people with pre-existing conditions, for instance, would reduce access. But if sick people sign up for insurance and healthy people don’t, that could trigger a death spiral.

Depending on how different members of Congress prioritize these objectives, they’ll almost inevitably wind up with multiple, somewhat incompatible ideas for a health care bill, and it can be hard or even impossible for a group to come to a consensus when it has more than two such alternatives. Republicans also face the additional constraint of needing to pass their bill using the reconciliation process — which limits what it can contain — or almost certainly facing a Democratic filibuster.

The other, related factor is that there’s a fair amount of ideological diversity within the GOP, even though that variation can be hidden on most votes. That is to say, Republicans lawmakers range from “conservative” to “very conservative” to “very, very conservative.” That might sound like a joke, but these are actually fairly meaningful differences. The statistical system DW-Nominate rates members of Congress on a scale from negative 1 to positive 1, where negative scores are liberal and positive ones are conservative. It’s fairly well known that Republicans have grown more conservative over the years. But the spread between the most and least conservative Republicans hasn’t narrowed. Moderate Republicans have been replaced by conservatives, but conservative Republicans have been replaced by very, very conservative Republicans.

Where Republicans all line up to one side of an issue — for instance, on tax cuts — these differences don’t matter much. The most fiscally conservative Republicans might want an especially large tax cut, for instance, but they probably wouldn’t oppose a small one. But on health care, to repeat ourselves, there are more than two sides, and every tweak that caters to one group’s preferences probably contradicts someone else’s.

There’s a contrast here to Democrats, who have also lost much of their moderate, centrist wing, but whose most-liberal members aren’t appreciably more liberal than they were 20 or 30 years ago. Their ideological range has narrowed — instead of spanning from “very liberal” to “moderate,” they now range from “very liberal” to “somewhat liberal.” That may make the party more cohesive, perhaps at the cost of other objectives, such as making it more appealing to centrist voters. Democrats also haven’t historically faced the same divisions between “establishment” and “anti-establishment” members of the party that Republicans have (although that may be changing). So just because Democrats were able to pull together enough to pass Obamacare doesn’t necessarily mean Republicans will be able to do the same to repeal and replace it. There just might not be any single Republican health care bill that can earn the votes of a majority of the House and Senate.