There are several possible explanations about why this is happening, one is if there is an increase in the number of cryptocurrency users around the world most likely the first crypto they will ever buy will be bitcoin, this happened to me and I am sure that I am not an exception, but as they learn more about the market they will decide to diversify their holdings and will buy altcoins making the market dominance of bitcoin to go down.
Another explanation is simply that many of the altcoins in the market were not worth their price in bitcoin and that is why they have been suffering a long decline, and finally it is also possible that many investors seeing that bitcoin showed signs of a bull run decided to sell their altcoin holdings and this caused the general uptrend in the market dominance of bitcoin.
Those could be the reasons why Bitcoin's market dominance has been rising at an exponential rate. Since it's the first cryptocurrency that came into existence, it has managed to gather a lot of attention within the mainstream world. All the eyes are within Bitcoin's latest developments as it could affect or benefit other cryptocurrencies in the space. Compared with other altcoins, Bitcoin has the most level of adoption among merchants, businesses, and individuals alike. Not to mention, its development and innovation is far superior than most well-known altcoins in existence today.
With the Lightning Network, several privacy solutions, and smart contracts (with Rootstock), Bitcoin is the true leader within the crypto space. That's why it's being largely used as crypto's reserve currency on the market. As Bitcoin's market dominance grows over time, altcoin's prominence within the crypto space will be greatly diminished. Even so, there will be a few altcoins that will survive in the future because of their dedicated communities and development teams (like Ethereum, for example).
Nonetheless, time will tell us whenever Bitcoin's market dominance will stay that way, or it'll be reduced as altcoins start to rise once more. Knowing that Bitcoin has always been the leader of the crypto space, should give us more confidence to buy and hodl it for a very long time. Just my opinion