I can make three conjectures off the top of my head:

1. Bitcoin has some sort of "first mover" advantage which gives it a critical number of investors/PR that allows it to keep trading high, and no other coin will ever be able to catch up to it.

2. Bitcoin is actually technically superior to other coins, and it won't be supplanted until whatever the special sauce is, is replicated.

3. Something is hinky in the Bitcoin markets, eg, Tether, and this explains its divergence from other coins.