The most simple explanation that I can think of is that people still fear BTC's current price but that the FUD didnt work so the weak hands wouldnt sell their coins. Manipulators moved to alts but the mass stayed at BTC. Maybe alts will continue to rise for some more months but they cant bring their customers what BTC is offering ATM. Not in safety, volatility, long term investment, scalability, adoption and I could go on. Coins like monero will never have world wide adoption under the masses because people dont feel the need to trade in all those altcoins. Altcoins are more like a casino ( 90% of them). Long term they cant outperform BTC and so they are only fit for short term trades. Long term BTC Will outperfrom them, lots of altcoins will die after a year, look the historical snapshots on coinmarketcap. Most of them are promising everything but at the end of the day they aint worth shit.  Only a matter of time before the majority of the altcoinbubbles will pop because whales are seeing that noobs wont pump it up further. I dont even look at alts. Most of them are just to bad in terms of development, price\quality reward or ROI.