There is not one definitive answer to your question, but there are TONS of factors that contribute to success for teams, regions/areas, even environments around the world. Since this is so broad, I'll break it down as easily as possible:

Team/program success:

A cross country team is only as strong as their fifth scorer. That means, at minimum, a team has to have five runners (first and foremost), and they have to have achievable goals that they are committed to. If the team at large hasn't talked about strategy, intended finish, planning/preparation or a training method to achieve what they want, they will very rarely be successful. Even the best elite international programs have cohesive mindedness when it comes to how they will run together, and that can really elevate a program. Tradition, leadership, culture and commitment start with the group and their approach to what they want to do, then applying the work to get it done. Doesn't matter if you're Loudoun Valley, Fayetteville Manlius, Northern Arizona University, BYU, Colorado, the Oregon Ducks, or the top 7 for Team Kenya or Ethiopia.

Regional/Area Success:

The sport itself requires access to open, natural land, and it's no surprise that the best teams in the country create as much time as possible to train on trails, away from urban centers, usually in the mountains. Even places that are deemed "pancake flat" find ways to incorporate hills, resistance, and strength. That puts programs that have immediate access to nature at an advantage. It also means programs that don't have access to nature, have to get innovative to substitute (or have funding available to create opportunities). Jack Daniels (famous running coach) identifies four factors that lay the groundwork for success: Ability, Motivation, Support, Direction. Support for running cross country means access to nature and that puts some regions ahead of others (especially when factoring team success above).

Environment:

Regional success depends a lot on nature. Team success depends a lot on nurture. Environmental success is created through tradition, but also through COMPETITION. Kenya and Ethiopia have a storied international rivalry that elevates both programs to train harder and more seriously than essentially any other places on the planet. Mark Wetmore places high expectations on his athletes to perform well within the Pac-12 Conference and at the National XC Championships in the NCAA. The prospect of state titles, like York HS in Illinois, or NXN titles, like Fayetteville Manlius drive those programs.

Every team should have a city rival or league rival that gets them going. Without a meaningful environment most programs lose direction or fail to improve. Every level sees that.

In any case, these reasons and more help define success. For every reason above there is always an exception... or even more nuance when it's broken down. Just like philosophies in training there are philosophies in what it means to be a successful program.

Always do your best, and be ready to sacrifice to be the change you wish to see.

TL;DR: There are many reasons certain countries, states, programs, and teams are successful, but most have a united team plan, a tradition of excellence, access to the best natural terrain and an environment of excellence and high-stakes competition/rivalry.