That's because certificates are all about authentication not encryption. There is no real reason to move away from RSA for authentication. The reason that TLS moved away from RSA for encryption is that it is awkward to do forward secrecy with RSA due to the slowness of generating new RSA keypairs. In practice you would want to generate a new RSA keypair every, say, hour on the server and then somehow get it down to the cryptography level for use. Totally doable, but a different way of doing things.
