some altcoins price growth are too slow though market and bitcoin is up at the same time, its not easy to those alts to increase their price for a short period of time.. if you get boring on waiting for your alts to rise then why not invest in bitcoin..
Yea some altcoins look too slow, but we should never worry, provided they have working product, they will rise again. We just need to understand how they operate, just like you advised that people sell their coin for bitcoin, these has been the practice and one of the reasons why alts are not growing.
Some alt investors have converted their alts to bitcoin for now, till the time bitcoin will go sideways. I am sure once they see a sign that bitcoin will go sideways, they will start pumping money back to altcoins, and this will also cause a FOMO buy that usually see these coins increase to over 1000%.