From a logical point of view I think it's this. Bitcoin over the last few months, let's say since beginning of November has increased less than 2x in price whereas ethereum is about 3x its price. I feel that the larger growth Ethereum had means that it's more in a period of consolidation whereas bitcoin is still clambering to get back to the price where it was previously. Also bitcoin tends to perform better in slow periods than ethereum, ethereum's growth is very burst like.