The key thing for me I think is market confidence. Alts only surge when confidence is really high, such as the back end of 2017. If confidence is lower, then people hedge their bets and buy BTC as the safest and least volatile crypto. The long bear market may have ebnded, but it still casts a long shadow in terms of the effect on investor confidence. It may be some time before confidence returns to really high levels and creates an alt surge. But BTC price impacts this, too. As BTC price goes up, confidence grows, and the higher it goes the more the alts look undervalued and more of a bargain. So I think an alt surge is largely confidence-driven, but BTC price is the main thing that impacts this confidence level.