Why haven't the earliest participants sold their Bitcoin
Do you care about that? There are many reasons behind it and you never know the truth, so it's none of your business.
There are various reasons
Most fall under the category of been unable to access their coins
Either through loss of password or death
While the minority are still holding.
In all I can only speculate since i am not one them.
You could say this is the right reason.
We heard some news from them losing bitcoins because they lost their initial phrases, passwords, dead people that are mentioned does make sense but indeed we all just speculate on their bitcoins at the beginning not moving maybe because of the above reasons mentioned.