I have a theory - I admit I may be wrong, but sounds reasonable at first.

2017 has bought a lot of attention on blockchain technologies, which may have had an impact on Steem. Hence the market cap increase - but when hype was gone, speculation went away, as well, stabilizing the currency on a lower level.

Then again, not sure if I'm right, but as you can see, in the last few months it was nowhere near to the late May spike.

I'm almost definitely convinced it was "just" hype.

It did good to the community, though.