Most Helpful Guy
You know, social media is a big deal for young people so this doesn't surprise me. I don't believe in blocking or deleting but the fact is that breakups can be really hard and if deleting you off social media makes it easier for him, then so be it. Let him mourn in his own way and move on. Maybe you two can be civil in the future, but right now it's obviously not possible for him.
Most Helpful Girl
Your ex probably deleted you because it's the easiest way for him to move on. No distractions.
As for why he hasn't deleted his previous exes, I don't know. Maybe he has now learned from the past that it's easier to move on if there are no reminders on social media. Or maybe he didn't care as much about his other exes and found it easier to move on - thus not needing to delete them.
I think it's better for things to end cold turkey like this. There's no room for lingering feelings or distractions. Even if you can avoid each other on snapchat, it's better to close the door entirely. Plus, even just seeing your ex's username works as a reminder. Better to be consistent across all platforms.