Hasura and Prisma have very different approaches and end-goals at the moment.
TL;DR: Very simply put, Hasura is postgres first and adds an access-control layer to make it possible to query from the front end directly.
The longer version:
Prisma is intended to be a GraphQL ORM to Postgres (and several other databases with a common API) whereas Hasura is more database first and intended to let you leverage Postgres features over GraphQL.
Hasura comes with and lets you use any postgres migration system and then gives you GraphQL on top of it. This is what allows Hasura to give you GraphQL over any existing Postgres database and application even.
Hasura also comes with an optional access-control layer that makes it feasible to be used by frontend clients directly. This access control is inspired from postgres's RLS but does not depend on or use postgres RLS and can be coupled with application logic/auth.