This time was supposed to be different.

In past election cycles, presidential candidates have stayed in the race until they’ve run out of cash. But Super PACs, which can accept unlimited sums, were expected to change that equation. “Wealthy patrons might keep their favorite picks aloft through independent spending,” The Washington Post speculated in April. Super PACs meant that the crowded field was “unlikely to result in the quick exit of the GOP laggards,” reported Politico in August.

But now it’s September, and the laggards are scurrying toward the exits. Scott Walker quit the race on Monday, following former Texas Governor Rick Perry out the door.

Why couldn’t their billionaire backers keep their campaigns alive?

One answer has to do with the mechanics of political fundraising. Individual donors can give up to $2,700 directly to a candidate. In the peculiar argot of campaigns, this is known as “hard money.” To raise it, candidates spend countless hours on the phone—flattering, listening, cajoling, begging—and fly around the country, offering personal contact at fundraising events in exchange for cash, or woo bundlers to round up the donations for them. It’s a grueling, frequently humiliating process. As campaigns grind on and exhaust their most loyal donors, these dollars grow harder to raise just as they’re needed most.

Super PACS, which are nominally independent, seemed an appealing solution to many candidates. They can accept as much as individuals, unions, or corporations care to give. A single donor can, with the stroke of a pen, write a check for more than a candidate has raised in his entire campaign. And, the Supreme Court’s naiveté notwithstanding, Super PACs have proven remarkably adept at working in tandem with campaigns, even when they are technically complying with elections laws. (And with the deadlocked FEC unable to enforce those laws, it’s not clear that many are bothering.)   

But it turns out that there are some things that Super PACs can’t do. Hard money can pay for the full gamut of campaign expenses, from hiring staff to purchasing printer toner to putting ads up on television. Super PACs can pay for television ads, but they can’t pay for campaign staff.

Perry and Walker were hoping to hang on for long enough to allow nominally independent Super PACs to flood the airwaves with supportive ads. But long before the first caucus, their hard dollars dried up, leaving them unable to make payroll.

Perry, for example, raised only $1.1 million when he last disclosed his tally on June 1, and had stopped paying his staff by the time he dropped out in early September. His captive Super PAC, though, still had $13 million in unspent funds, tantalizingly out of reach. It transferred it back to donors, primarily Texas businessmen Darwin Deason and Kelsey Warren.

There’s another price candidates have paid for their reliance on Super PACs. Candidates want their most trusted advisors in charge of these nominally independent entities, to ensure that they serve their interests. But once they take the helm of a Super PAC, they’re legally barred from offering the trusted counsel and honest feedback that earned them their positions.

As my colleague Molly Ball recently pointed out, Walker installed Keith Gilkes, who managed his campaign in 2010 and the 2012 recall election, and Stephan Thompson, who headed his 2014 reelection bid, at the head of his Super PAC. Walker was fond of saying he’d won three elections in four years—but he was walled off from the two men who’d helped him do it. The result was a campaign that was not just underfunded, but also poorly managed.

But if the impact of Super PACs has been both widely exaggerated and more mixed than expected, they may still end up playing a critical role in the 2016 election. They haven’t relieved candidates of the need to raise hard-money dollars early in the primary cycle. But as the field narrows, and the campaign’s focus moves from state fairs and VFW lodges to the airwaves, billionaire donors may yet have an outsize impact on the outcome of the race.