The routes on Kilimanjaro vary by length, duration, difficulty, scenery, altitude profile, accessibility, and (lack of) facilities. The better a route scores on all these criteria, the more popular it is.
Popularity is great if you like to share your experience with many other climbers and make lots of new friends. It’s also great if you’re looking for an affordable climbing package. If solitude and wilderness are what you’re looking for, then the most popular routes may not be your best choice.