It's a Run & Walk, not a Run/Walk
Some events attract far more walkers than runners. Even those catering exclusively to runners often include a walk to allow participation by those who can't or won't run the distance.
When races do that, they are actually offering two events - a run and a walk - and keeping them separate is an important safety issue.
Runners are traveling faster and are competing, even if they are slow (slow runners are usually as serious about racing as are fast runners). Split times and finishing times are an essential part of the race.
Walkers are participating in a non-competitive event. Timing is a courtesy, not a necessity.
Walkers frequently travel in groups, span the entire course, and often refuse to move aside for runners. Runners have no choice but to charge through these groups, endangering the physical welfare of each.
To a runner, the term "run/walk" suggests mixing of walkers and runners and little concern for race standards.
Calling the event a "Run & Walk," and requiring separate registration for each, allows race directors to produce and promote both a quality run and a quality walk.
Runners and walkers are two very different markets. You need to satisfy both of them.