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.
Yes, bitcoin was not the first crypto or the first decentralized currency, but it was the first successful crypto. Before bitcoin, BitGold and BMoney were the first decentralized cryptocurrencies but both were just theories, they were never fully implemented so they quickly failed.
To be fair, it is difficult to see the real reason why XMR or ZCASH cannot become popular even though they are completely superior to bitcoin in terms of privacy and anonymity. But one thing is that they have become less attractive and less competitive from 2018 - 2020 as the US and EU governments have stepped up their crackdown on them. If the government doesn't crack down and stop them, and people don't just care about profits but also prioritize privacy and anonymity, it's hard to imagine what will happen to bitcoin.