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.
Those investors especially the big whales are the one making this coin to be more popular and also the features of this technology is really the biggest factor why it is becoming popular. Moreover, more and more people are now started to adopt the technology and hopefully also it will be legal to all countries.