Grammars were great for social mobility when there were a lot of them.
Once privately educated Labour leaders demanded their abolition, the remaining grammars became more middle class, being scarcer and better providers of education. Middle class people are better at working the system.
So, grammars lost their value as providers of social mobility (except in regions where they had not been abolished wholesale).
The middle classes, frequently declared socialists, some serving in Labour governments, continued to use private schools and grammar schools for educating their own children, while simultaneously declaring that "everyone" (trans. other people's children) should have an egalitarian education in a comp.
Thirty years pass. The results of comping are visible to all but the most deluded. Tories decide the oiks need to have a chance again. Cue howls from "egalitarian" Labour. Rinse, repeat.
This country conflates education with class. It is a GIVEN that bright children from homes without educational resources and support need a chance at life. We understand this with sport. It is a very small leap to extend opportunity to education, yet Labour always howl it down.
I genuinely have no idea why, but it is the single reason, as a very bright oik, that I'll never, ever vote Labour.