Enforcing a limit would likely do the opposite - and make the convoys spread further apart.
Most convoys have a "lead speed", usually no more than 90km/h that the lead of the convoy always sticks to. What happens though is people crash, or lag out, and a gap is formed. In order the close the gap and keep the convoy together, fast speeds are sometimes required. If there is a lower limit, it would be harder to close these gaps in the convoy.