Haram means illegal, I don't see any special and logical reason behind the bitcoin is illegal. Day by day it's gaining popularity among the people who engage in virtual work. So i 100% believe that its is totally contradict the people idea about haram of bitcoin because its 100% halal (legal).
Bitcoin is a currency.
Dollar is a currency.
Pound is a currency.
Euro is a currency.
All other fiats are currency...
Dollar, Pound, Euro and all others currencies are Haram?
I leave you to justify your statement yourself.
Though the Dollar, British pound are not haram/illegal, there is quite good reason for that. First reason is US dollar, british Pound are recognized by their respective country as a legal currency. Same is true for currency supported by other countries. Every country has their own currency. This is where bitcoin or for that matter cryptocurrency differ from other. These are not backed up by other govts(mostly). Many of them find it encroaching countries right to control economy.
To sum up, in purest sense bitcoin or cryptocurrency is not haram/illegal. Its legality totally depends on country in which you reside in.