What's strange about this bull run is when a pull back happens, the alts are not rising. They're also dropping in unison.
A handful of stronger alts have performed well vs. BTC.
In general though, you're right. I believe that's happening for 2 reasons. First, BTC momentum is too strong. There has been no sustained sideways consolidation for the last few weeks, so there is immense pressure to exit alts and hold BTC. Second, I think retail has barely begun piling into the market. Everyone I talk to already thinks BTC is "too expensive" though, so when they do FOMO into the market, they'll be buying alts.