Sure, they could have - but didn't. Probably because they were strongly biased toward nonces being generated by a counter, and separating an external 'message counter' from the internal 'block counter' is a nice scheme to have essentially unlimited, large messages without any concern for nonce reuse.

Except for subsequent experiences showing us that reliably keeping track of a counter can be a lot harder than it first appears, hence the attractiveness of randomized nonces.