When you make a promise and think that nothing in this world can break that promise, you believe it.
People say that long distance relationships never work, but did that count friendship as well? Well, they did, but no one tells you that part. Having a best friend that is both far away from you and for a long period of time is tough. You don't experience the same things every day. No more random ice cream runs. No more walking into each other's houses and going straight for the fridge. There are less and less inside jokes. And just, less time around each other.
Two separate people living two separate lives. You live your life, make new friends and they do the same, but everybody grows and changes differently with or without each other. In this case, being far away and out of reach makes it where people grow differently. They could still be the same person but developing into a different direction from you, and that's the scary part.
Don't get me wrong, this doesn't happen to everyone, just the ones that decide not to stick around. People simply grow apart and away from each other. Times change and so do people, but it happens.
They become "too busy" or have "no time to hang out or talk" but come on, "too busy" for your best friend is someone that probably doesn't deserve your time and energy anyway. If they can't realize what they are missing out on, then that's their loss and your gain. Plus you could take a nap instead of spending time with them while they're on their phone "being busy".
So keep the ones that matter, and let go of the ones that don't
In the end, it happened for a reason and they were not meant to be apart of the next chapter of your life. On the bright side, it opens up more room and opportunity to find who cares and may be your new "person".