The more mature a market becomes the "safer" it is to enter. People that thought the market was not ready at 4k may think its good to go in now.
This is what you see with the big institutions as well. They need liquidity to put their massive funds into it. The higher the price, the more room there is to buy big.
The institutional investors have mopped up a lot of the available supply during the last few months. This is beneficial for the other investors, because these investments reduce the overall volatility. Institutional investors such as Grayscale are unlikely to dump their coins, and this means that the chances of a price crash gets lower. Also, these investors can be expected to store their coins more securely when compared to the institutional investors. A lot many times, corrections occur when hackers dump their stolen BTC in to the market.