(It's Thomas). Because it would have made even less sense to define a TLS-specific heartbeat and a DTLS-specific heartbeat.

In the hierarchy of sensible TLS decisions, you have, from most to least reasonable:

1. Not adding new heartbeat extensions to DTLS or TLS.

2. Adding new heartbeat extensions to DTLS only.

3. Adding the same new heartbeat extensions to DTLS and TLS.

4. Adding two different new heartbeat extensions, one for DTLS and the other TLS.