I'm guessing it is because people are now buying coins with a higher market cap. They bought lesser known coins during the last bull run hoping to make greater gains, but ended up losing a lot more money than if they bought coins with a greater market cap.

It's either that or altcoins haven't had their run yet. We all know most cryptocurrencies are tied to BTC, not that the developers choose that, but it is just how the market goes. This tie is sometimes delayed and even the bigger altcoins haven't had such a great push yet. If it is this case, I think we just have to wait.