Bitcoin has always been a leader, and mostly because it was created first. This is an advantage that no altcoin can overcome. And in fact, I do not know a single coin that would be better than bitcoin. And that's what most people think, as we can see.
That's certainly true, mate. Bitcoin has been trusted for a very long time, since it has been the world's first cryptocurrency with a proven track record of stability and security. When you compare other altcoins against Bitcoin, none can match its censorship-resistance and resiliency. I believe that the reason why market dominance has been rising so fast is because people became tired of most altcoins which made lots of promises without delivering real use cases for the mainstream world. Only a select few altcoins are extremely prominent with active development and innovation. Still, they're not comparable to Bitcoin because of many obvious reasons.
The more Bitcoin rises in dominance, the better it'll be as altcoins that are useless will fade into oblivion. Every day, altcoins decline in price at a faster rate than Bitcoin. Right now, Bitcoin has retained is value while most altcoins are even cheaper than they've used to be. In the end, we'll be dealing with an extremely limited number of altcoins on the market (like Ethereum, Cardano, EOS, and Litecoin) while Bitcoin remains as the "King of Crypto" for perpetuity.
Nonetheless, it's hoped that Bitcoin's market dominance will rise even further with the introduction of a stable version of the Lightning Network + Atomic Swaps. Only then, most altcoins will be rendered useless, greatly contributing towards the demand of Bitcoin for years to come. Just my thoughts