Antiviral chewing gum to reduce influenza and herpes simplex virus transmission

Multiple reasons.

The first one is basic ethics. Similar on how you should do rolling upgrades of your SaaS software to catch errors before everything goes down, you got to do the same with vaccines. For there, go for the target group with the highest risk and highest potential of averting damage - and for the HPV vaccine, mid-20 women are the best such group: young enough that they might be lucky and not exposed yet, old enough to fall out of the scope of the usual ethics bureaucracy that (rightfully) comes with doing experimental research involving minors, and not so old that they definitely got exposed and making the effort moot. Then it got rolled out to teenage girls as it was proven safe, and eventually to men as well because we can be asymptomatic carriers (as we are for a lot of STDs).

Obviously if you got the speed of Covid vaccines in mind as a comparison, the HSV vaccine appears slow in rollout speed - but please do not forget, the Covid vaccines went through very speedy trials. We were extremely lucky it worked out the way it did.

The second one is availability. Again, unless it's Covid where everything went into full production power in a matter of months, production has to be ramped up carefully, matching rollout strategies - it doesn't make sense to have a mismatch into either direction.

And the third one is time. With Covid, it was easy to prove effectiveness: the people that got the shot got Covid at waaaay lower rates than the control population (and the risks of side effect were way less than the risk of severe Covid). But with something like HPV that can have years if not decades worth of time between exposure and symptoms, it becomes harder to reasonably judge effectiveness and safety.