It was the fact that he was slower than most of the offensive linemen that made him a non-existant running threat. Teams didn't have to blitz. They could simply play 8 men in a short zone, and take away most of the offense that Rees was capable of running (short passing game).
To compare Trevor Lawrence (who is a much superior athlete) to Tommy Rees in terms of running ability isn't a fair comparison. Trevor Lawrence can at least run around a 4.70 in the 40 whereas Tommy Rees was about an entire half second slower, not to mention, being physically stronger by a good bit.
To put it this way, when a football athlete gets caught by a slow-footed police officer, that should say something about his running abilities (or lack thereof).