The older cars were not claimed to be "clean diesels"
That "clean diesel" marketing claim is 100% of the reason there's buybacks and payouts. The payouts and buybacks are 100% driven by the FTC, and NOT the EPA.
The EPA scandal would not have been nearly as consequential to VW if they had not marketed the CR cars as "clean diesels" in order to achieve maximum market penetration. The cars were touted as "prius alternatives" that are actually fun to drive, better cars that are not a tin can but still environmentally friendly, and so forth. VW would not have sold nearly as many as they did, without the marketing as "clean diesels". When the EPA findings came out, then came the "deceptive marketing" claims... hey maybe my "clean diesel" isn't so clean after all like VW claimed, I would not have bought it if I knew, I'm worried about bringing my kid to school in a toxic car.... blah blah. The consumer buybacks and payouts are what makes this so significant and those came about as a result of the FTC action, and the EPA only indirectly.
Sure, the EPA still had their day, and not meeting the stated emissions standards was important, but that in and of itself was more akin to a "breach of contract" with the EPA, and could probably have been negotiated to a recall and fine.
The payouts and buybacks resulted from a settlement with the FTC, which came about as a consequence of the EPA findings. But to most people's minds, they just jump over this detail to say "VW is paying because of the EPA." But when it comes right down to brass tacks, it's actually "VW is paying because of the FTC".
there are no fraudulent marketing claims for the older cars. So no FTC lawsuits, no consumer fraud, no payouts or buybacks.
And like IBW said, the regulations have become progressively more restrictive over the years. For example, VW could have chosen a less strict standard but doing so would risk 50 state certification, much like what happened with the PD cars. But that didn't fit with the goals VW had for market penetration of their "prius alternative" so the rest is history.