Well simply because it has what other cryptocurrencies do not have which is time
bitcoin was the first ever cryptocurrency ever made let’s just say that bitcoin was not slow and expensive back then but of course time had passed and more and more people have discovered it hence increasing market activity but it’s still the #1 because it is what has proved itself against time
Time? Hhmm, that's odd. I thought time only exist for the living things like for us humans?
We don't just say, but it is actually true that BTC were fast and inexpensive in its early days. Though, the reason you have given there is correct and responsible for its change. Not just in Bitcoin but if we think about other things, they also work the same; that if more and more people are using them, there is also a slow down that happens.
And for it to have a remedy, we are required to pay a little more. People needs to start realizing this, so that they will stop complaining about the high fees in BTC sometimes, or whenever the network is fully loaded.