It's a little bit hard for me to explain why Bitcoin holds the leadership despite the fact that there are other competitive cryptocurrencies like Monero. Since 2016, when I first interacted with Bitcoin and cryptocurrencies, I've always believed that Monero would be the next Bitcoin because it has everything that Bitcoin has but in a more polished way, and Monero even does more than Bitcoin can. Logically, Monero should be the number one cryptocurrency because Monero has stealth addresses, ring signatures, native Tor support and etc... Monero has the highest privacy, the most decentralisation, and it's one of the safest currencies.
Why is Bitcoin still the leader and not Monero? Does anyone have any explanation?
I think the first advantage is because Bitcoin was the first decentralized cryptocurrency and had an absolute community support before Monero came along (bitcoin maximalists).
Second, bitcoin was designed to ensure transparency and security, rather than prioritizing absolute privacy (complete anonymity), and not everyone appreciates the absolute privacy Monero offers. There is also another factor that I think many people will be interested in, which is supply, Bitcoin has a limited supply while XMR does not.
Lastly and most importantly, what most investors care about is returns and bitcoin has clearly been doing well in this aspect before XMR came along. So it's no surprise that people continue to put their faith in bitcoin instead of XMR.
I don't know if Bitcoin was the first decentralised cryptocurrency but I know that it was not the first cryptocurrency, there were some attempts long before Bitcoin but Bitcoin succeeded. Satoshi even said that in a few years, Bitcoin will either be very popular or dead. I also remember that Bitcoin was promoted for being private, not transparent. Bitcoin mixers were very popular too, Bitmixer was mixing millions of Bitcoins every year. I still don't have a solid answer to the question why XMR didn't succeed, it was created in 2014 and it's not late for a cryptocurrency. Zcash, which was created in 2016, was interesting too but its price fell from $2000 to $50, many had high hopes.