They've never been particularly low-power, in a performance-per-Watt sort of way compared to other offerings at whatever present day. In recent times, I've seen completely-believable reports that N150 boxes walk all over it.

They've never been particularly cheap, in a performance-per-dollar sort of way. Used machines from eBay, yard sales, and old broken laptops (that still compute!) have always been better. (They usually come out OK when new is compared to new, though, which is IMHO the only valid comparison.)

Those comparisons were never very favorable.

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The parts where it shines are: Small; they come in two sizes, and those sizes are small and smaller. That was new, but it's been cloned all over the place.

Built-in GPIO that's meant for people to actually-use and tinker with. That was new, too, but it's also been cloned. (Also: These days, anyone can plug a cheap Pi Pico into any PC with USB and get a fairly intense amount of GPIO to goof with.)

Standardization, and the appliance-like behavior this can enable. Lots of folks, including kids of all ages, just download pre-built images and swap SD cards like they would cartridges in a Nintendo. That's not for me, but it's pretty neat.

Community. They've still got a lot of momentum by being first at these roles. That's good. It helps newbs (who at this point may have never had anything resembling a "real computer" to play with ever before in their lives) to get started.

Low-power enough: It's not ideal when chasing tiny Watts for battery or small solar power, but you probably won't notice it on an electric bill (and despite the bizarro-world cooling rigs people put together, a passive heatsink really is good enough to keep it running in-spec).