Banking institutions, law enforcement agencies, business leaders, Governments and many others call Bitcoin illegal.
But then - Why is the use of Bitcoin increasing?
Not actually the use of Bitcoin, but the investors are the ones increasing. To be honest, until now there are less people using bitcoin as payments especially when the Silkroad has been shut down by the "man". On the other hand, bitcoin isn't illegal... it's just sort of unrecognized... but you can wait for a year or two and let's see what will happen.
And if (or right to say "when") Bitcoin will become legal, what will do the community? Will HODLers start to spend their coins or hold and buy more bitcoin. As we know, BTC network have some problems (transactions frequency, high fees) that must be solve.