The real question here should be, why shouldn't bitcoin dominance be increasing when the so called altcoins don't have what it takes to give bitcoin a fight for it's dominance. 99% of the current Altcoins whether in IEO/ICO, already trading or in development stages doesn't have anything new to offer to the industry. They're just false hope gaining attention based on hype. Bitcoin is a working product with years of experience (10 to be precised). What then do you expect. The community is putting their trust and money in the right place that's why the dominance is increasing fast.