As with everything in Crypto, there are highs and lows. Bitcoin dominance is going up once again but I highly doubt we'll ever see it approach its previous levels of dominance.
We have to remember, Bitcoin is the oldest, and some can argue, the most secure Cryptocurrency out there. Because of these reasons, people think it's safe to keep their money there, especially when it looks like it is about to enter another bull run. But typically when the BTC bull run ends, that's when we see the alts start climbing up again.