Realistically they compete with Neustar which is shockingly expensive and has less features and is harder to use.

I chose Dyn over Neustar (UltraDNS) when it was time to renew contracts because it was 60% cheaper, had a better latency, their support was great and the interfaces were clear.

Not a fanboy or anything, I really don't like how aggressively they hound me now (even though I have nothing to do with DNS for my current employer), but it's cheap and effective so it's not surprising people use them.