Because your entire premise here sort of betrays a fundamental misunderstanding of the concept?
So we stop vaccinating people here, and require everyone that enters be vaccinated. Problem solved!
Except not really, because people here travel to foreign places. So also everyone that _leaves_ here needs to be vaccinated as well. But that's do-able.
Except, and here's the part where "herd immunity" is important: Being vaccinated doesn't prevent contracting the disease 100% of the time. The reason vaccines are effective isn't "you can never get the disease", it's because "if you get it, it can't spread to enough people to sustain itself into an outbreak".
If you require people traveling into or out of the country to be vaccinated and stop vaccinating at home, you're going to have someone bring polio back fairly quickly. The vaccine is not 100% effective, and polio is asymptomatic in about 75% of cases so many carriers wouldn't even know they have it.
In a vaccinated population, that's not a problem. The effective reproduction rate is In your unvaccinated population, the R0 is 5-7. For every person infected, five to seven others will be infected. The growth will explode quickly and given the rate of asymptomatic carriers there's no chance you're going to contain it.
(Also have you ever watched any zombie virus/fungus/etc show? Even if the vaccine is 100% effective, do you really want to hang the health of the whole country on nobody ever messing up vaccine administration, nobody lying, nobody faking paperwork, etc?)
So everyone has to be vaccinated anyway, and if they're all vaccinated anyway the risk of letting a potential carrier in is... pretty near nil in the grand scheme of things.
Your options are:
* Never let anyone in or out. (But honestly, it'll still probably get in. See: lying.)
* Eradicated the disease globally. We can do it, but less likely to happen now with USAID shut down.
* Vaccinate at home so when the disease gets in the country it doesn't spread.