Bitcoin has been steadily losing market cap dominance, from 90%+ just two years ago to ~80% today.
When it comes to exchange dominance it's just 66.4%.
This trend will probably continue and in a few years there will be people using cryptocurrencies that do not know what bitcoin is.
People trade for money. Trading Bitcoin, which has been quite stable until the current pump, was boring, there had few money to make, so they headed to altcoins market because it is cheaper, so even a little rise, combinated with possessing 100 000 of this altcoins will make great bucks in your pocket.