altcoins have grown much more than bitcoin in the last 3months.

That just confirms my theory. A lot of people have sold out their bitcoins in the last 3 months expecting some meaningful price correction that never happened.
Wait, are you implicitly saying something like: "They made a big mistake, they sold BTC for altcoins which gave them greater profits."?
Just wait for the big dump in alts, and huge pump in btc. It's all about bitcoin(blockchain)