Dimetrodons were pelycosaurs, which was an early synapsid group flourishing around 290 million years ago, during the early Permian. The therapsids evolved from them or from similar synapsids (and were also synapsids themselves since the name refers to the skull structure). Dimetrodon was probably not endothermic (the whole point of the sail was almost certainly as a combined heat-absorber / heat-radiator depending on which way the animal stood relative to the direction of the sunlight), but the therapsids almost certainly were.

My point is that the sail-backed pelycosaurs, because the sail gave them a superior control over their own body temperature compared to other reptiles, may have evolved more efficient but more temperature-sensitive metabolic features, which in turn created an evolutionary "demand" for true endothermy as a way of supporting those features without growing the energy-costly and awkward sail. The therapsids also evolved to fill a smaller-animal niche; obviously a big obvious sail is not an accessory that any animal which needed to burrow or hide could afford to carry!

Mammals are anciently burrowers; perhaps one reason is that the original definition of a mammal may have been "a pelycosaur without a sail!"

- Jordan