Bitcoin is still illegal in some countries for many reasons, I think thats because some of these countries are having trouble to control the bitcoin. because its uncontrolled and there is a possibility for a new way for crime activities, those are some other reasons that I often hear in the media.
I think that the only reason can be that most of the countries still do not have enough knowlege about bitcoin. I am sure that with the passage of time when all the big and develeped coutires will accpet bitcoin, then they will also consider bitcoin as legal currency. In fact bitcoin is the main currency of the modern time and all the nation have to accpet it as legal currency.