Because altcoin world is too large comparing to single currency on top BITCOIN, how can you speculate a thousand currencies when bitcoin is just there always to show what is the movement in front of us.
and besides why need to expect in altcoins when there is bitcoin that safer and worth trusting .
There are lots of current events affecting Bitcoin and most altcoins rely on it. It's really hard to speculate altcoins because of their volatility and that's its nature. However, there are good coins that we could speculate with smooth accuracy simply because we already know how to deal with its volatility. If you already know a lot about a certain coin, it would be easy for you to deal with its movement and could be considered as an edge.