It is because Bitcoin has a huge community compared to Altcoins and of course the adoption around the world. It is also the most popular cryptocurrency to date and crypto is nothing without Bitcoin. Some country even accepted Bitcoin as a valuable asset and can be traded locally.
I don't think that Bitcoin has that bigger community than some popular altcoins. Many of top 10 CMC projects have really big community and I would not be surprised if one of them is even larger than Bitcoin's community. I'd say that the biggest reason why there is no altcoin that can replace Bitcoin is because we are still very early in the crypto adoption. Bitcoin is the king of all crypto and it probably will be the king for quite some time. But that doesn't mean that there won't be some altcoin that will emerge and dethrone Bitcoin one day.