Because it is the easiest investment to have?
It could give you a huge profit if played right.
Easy to buy or sell anytime you want.
You could hold your own investment if you want (By putting it on your hardware wallet).
This gets rather not true as time goes by because a lot of people are making bitcoin much more complex but if you still do the old way, then you can probably still do it but some companies entering bitcoin are making things more complicated. But overall I do agree that it's an easy investment with a really big returns and it's unlike any other existing out there.