Because:
Consumer iMac: 24”
Prosumer/former iMac Pro/larger iMac: 27-30”
Exactly.
Apple are true masters of making you feel the "step up" when choosing a more expensive model over a cheaper one.
The "step up" between various product models is one of the key factors to Apple's convincing us to happily sacrifice $999+ on an iPhone Pro even though the flagship iPhones were priced $849 not too long ago.
It's as basic as looking at your local fastfood joint that offers you three to four sizes of soft drinks -small, medium, large, and extra large "big gulp" thirst-quencher extreme: Dollar-for-dollar, you get way more if you splurge on the latter in comparison to small, medium and large. No, you probably don't need or can ever consume the biggest size, but because of the wording and the, comparatively, lower value you get by going small, medium, or large, you end up going for a larger size just because it's the smaller options make you feel like your losing more or not gaining as much as going bigger.
-The 24" iMac is very deliberately large enough to be attractive to most yet not quite big enough to be interesting for prosumers or those that come from 27"+ displays who also watch movies on their iMacs. The smaller displays, lesser specs, and the glossy, plastic-ey color options of the 24" iMacs will serve to highlight the added value of the coming 27"-32" iMac Pro, with bigger displays and metallic "indstrutrial" looking designs and color options, probably also smaller bezels.
Conversely, a 27"-32" inch iMac Pro would be a much harder sell if Apple had already launched a 27" iMac M1 together with the 24" M1 iMac.
Apple knows that most consumers only recognise and grasp the size of a display and not things like panel type or pixel density, so the vast majority wouldn't understand why they should get a 27" iMac Pro at $2500+ if there also was a 27" M1 iMac at <$2000.