Bitcoin's share of the total marketcap (not its dominance) is decreasing because of a very simple math.

market capitalization = supply * price

increase the supply you increase the marketcap.
increase the supply to a ridiculous number like 100 billion you get a ridiculously big marketcap.
now have hundreds of altcoins with ridiculous numbers you get a huge total marketcap in which bitcoin has a tiny share.

Manipulation 101