It is difficult to give the reasons in one or two bullet points. These are the reasons that come to my mind right now:
1. There was no pre-mine for Bitcoin
2. It is the original cryptocurrency
3. The inventor (Satoshi Nakamoto) has never tried to profit out of BTC
4. None of the coins come close in adoption and acceptability when compared to BTC
5. BTC can be traded with a large number of options (fiats & other coins).
I’m just realizing that Satoshi Nakamoto never tried to profit out of Bitcoin. That’s unlike other people that created different coins like Ethereum and the rest, because I do know for sure that their owners have at certain times profit from it, just like the founder of Ether and his Ether foundation.
Really Bitcoin transfers are expensive and lower speed, therefore altcoins are much easy to transfer. For small transfer amounts Bitcoin is not usable.
I think people make use of what they like and not really what seems to be better. Bitcoin is the first cryptocurrency and people are becoming used to it and trusting it more than altcoins. I do know that Ethereum has a cheaper fees and also faster when it comes to transaction, but still I hardly make use of it. I have preference for Bitcoin and I can’t even give reasons why, and I do feel it’s more safe.