In the firs place, bitcoin experienced a good growth rate in 2017. The drop in December is insignificant for me considering the rise from 1200$ to now around 13000$. Why altcoins getting better place on market? First Developers are doing a great work and secondly some people think it is a good idea to invest in altcoins which haven't experienced big growths. Imagine using 12000$ for 1btc while the same can give you over 1000000 0f an altcoin:D
despite this, I still choose btc as my primary wallet coin to hold