Ether is so easy to use, has a team, has technology, has a road map, has a good community, has many features that Bitcoin doesn't have, why not support it? This is obviously a better choice, Bitcoin is too old.
Bitcoin is still the one dominating the market simply because it is the most popular cryptocurrency. Some people even think that bitcoin is only the cryptocurrency that exist. Besides that, bitcoin has more volume than ethereum so it is easier for those whales to trade bitcoin than ethereum. People want to overcome BTC because they can buy more ethereum than bitcoin so if ethereum would be able to take bitcoin's position then it would mean its price increase and people would have more profits. I like to use Ethereum in sending coins than bitcoin because it has cheaper transaction fee but at the moment, it is pretty expensive.