You have to see that Bitcoin has a greater number of users and many of them are big investors, so they analyze with more care the market trends or the changes that may occur, on the other hand, the altcoins still do not reach the level of participation high that is expected.
I agree. Bitcoin is most popular and many investors are focus on investing with it than altcoins because its potential is already there where nothing to worry for the success of it. They believe on bitcoin rather than altcoins because more of them never gain success unlike bitcoin that every improvement give it a success.