People are investing in other currencies rather than bitcoin because
a. its very costly nowadays to buy one whole coin
b. its not very suitable to trade such a huge amount

that being said people are investing in steem because you can get more money out of it even without spending the initial investment by SP steem power

steemit pays us on upvotes / comments / posting

and people who have large amount of steem power can also upvote themselves and get huge chunk of rewards just by themselves

that's what the whales do here (people who have lots of steempower)