The heartbeat extension was originally designed for DTLS, which is TLS over UDP. UDP datagrams don't arrive in a guaranteed order, or within any bounded time. If heartbeat used a fixed message, then if you sent a request and receive a response using DTLS, you would have no way of knowing if that response was a response to that request, or to some other request you sent ten minutes ago, or what.

Allowing the request and response to carry a payload lets you mark each request with a unique identifier, so that you can recognise the resulting responses.

It's basically like an HTTP cookie. In reverse.

I have no idea why the payload is variable-sized, rather than being (say) a fixed 32 bits. I would guess the specifier didn't want to bake in assumptions about how much space a unique identifier would need.