All soft forks impose new restrictions on miners that they have to follow to stay in consensus. E.g. the segwit softfork took bc1 addresses -- which used to be "anyone can spend" addresses, because you could spend them without a signature -- and made it so you could only spend them if you put a signature in the witness data. At the time, miners running an old node still "thought" they were anyone_can_spend addresses, but if they had ever tried creating a block spending one without a signature, upgraded nodes would have considered that block invalid.

So when a soft fork happens it is important for every miner to update their nodes otherwise they can mine invalid blocks and the network will reject them...and they will lose money