It's exactly that. The baggage rules that have come about in recent years is all about enabling quick turnarounds and good on time performance. As much as some may want to think it's about squeezing a few extra quid out of people are very much mistaken. Delays have a far greater cost to the airlines than any extra revenue squeezed out of oversized bags is ever going to yield.
Historically, the low cost airlines have charged through the nose for checked bags as they simply didn't want them. It added cost and complexity to the operation. However, as the industry has grown, more airlines are flying into congested slot constrained airports and the daily occurrence of air traffic flow delays in summer, the airlines have had no choice but to do everything within their own power to enable good on time performance and minimize any potential for delays.
With that the whole "we don't want your checked bag" ethos has gone full circle and now the airlines broadly encourage checked bags over cabin baggage as the latter has more potential for delays. Ryanair for instance has reduced their checked bags fees over the years and increased from a traditional 15kg to 20kg allowance. The likes of Jet2 and TUI for instance will still allow a standard 10kg cabin bag for free because they have longer turns and many of their customers will have 20-22kg checked luggage as part of their holiday package anyway, so the demand for cabin bags naturally reduces.
The low cost carriers are trying to replicate that to some extent with their own pricing/policies. It's all about driving the behavior of customers to suit the needs of the operation, not simply a means of squeezing extra revenue.