One factor would be that there a no new investors for bitcoin. These are mostly traders who trade bitcoin to altcoin and vice versa. Meaning old holders of altcoins that switches to bitcoin to hold.
This is not true at all. There are many cash flowing in to bitcoin market even in the recent month from big players. You should read the news about this kind of things more because the main reason why bitcoin is so bullish right now is because many investment firm decides to diversify their portfolio in cryptocurrency more specifically bitcoin which means the market got opened up to much much more liquidity and increase in market capitalization.