Originally, probably because it was inflexible. You have to use the in-memory layout required by the hardware, and you can't do anything not supported by the hardware. Later, it didn't get updated (probably because no one used it) so today it won't save things like the MMX and SSE registers, which makes it even less useful.

It's certainly much faster today to use sequences of mov instructions. In practice, it's a block move of multiple registers to memory/cache. That's something any modern pipelined and microcoded processor will have specialized support for. What instruction or instructions generates that internal action, is largely beside the point.