Many people sell Bitcoin and crypto in general because there's still many doubts about it.
They are scared that crypto is a balloon that is about to pop any time soon and they want to secure their profits/investment.
Crypto is still in early phase and isn't mass adopted yet, so I understand where the fear might be coming from.
I think people sell bitcoin not just only because of the lack of trust, because i think most of people know that for them to earn and to gain profit with the used of bitcoin they will need to sell it and used it in their own local currency just to fell that they formally have a profit out of their money or bitcoin. But in realty is not bad to deal with this because transaction could also help the community of bitcoin to send awareness for other people to know that they could also earn with the used of bitcoin.