It is because simply bitcoin is the first mover advantage. When it is the first, it would naturally be also the first to be spread around. When would we have learnt other coins before knowing about bitcoin.
2ndly, bitcoin is one of the most well distributed coins out there. Even now with thousand of different coins out there, one common problem all these coins faced is that there is no one coin that can reach out to such a wild variety of people out there.
The level of exposure, when you talk about crypto in local news or media, you will always heard about bitcoin first. So it already have became a stagma that bitcoin is the leader and this kind of mindset will take a long time to change.
Bitcoin is one of the most decentralized coins. Even for top 50 coins, they may not be as decentralised as the coins will have certain co-founders having significant control over it. As for bitcoin, there is not really a singular sources that is able to dominate over it. So its power/admin/rights are very well distributed. For example, eth has well known co-founders to determine the development of the projects. We do have zcash facing a 51% attack because it is not well diverse enough etc.