A couple of things:

The cost of cryonics is more than just the liquid nitrogen. You need to mobilize a team to properly preserve the brain, then keep it in a refrigeration unit indefinitely.

If you keep tissue at temperatures slightly below 0ºC, it's not really frozen. Tiny pockets of concentrated ions will lower the freezing temperature of water in in those areas, keeping portions of the tissue liquid. I think the effect is similar to salting roads in the winter time. Anyway, the tissue degrades over time scales we care about.