Fast card support helps only if card writing is the bottleneck. In many DSLRs, it isn’t.

Burst shooting is limited by several stages: sensor readout, image processing, file creation, buffer size, and then card write speed. The camera’s internal processing pipeline often caps frame rate and burst performance before the card does. Faster interfaces also require more capable hardware, which adds cost.

That’s why many cameras don’t gain much from UHS-II or other very fast card standards in normal use. For example, some DSLRs can already sustain their maximum JPEG burst rate with UHS-I cards, so faster cards mainly benefit photographers who shoot long continuous bursts in RAW.

That’s a narrower audience, mostly action/sports/wildlife users, so manufacturers have tended to reserve the fastest storage options for higher-end bodies where that demand is strongest. In short: slow adoption is usually more about overall camera design, processing limits, and market demand than a deliberate attempt to hold back card speed alone.