Governments can easily legalize bitcoin by passing a law, but I think it would be difficult for them to control bitcoin transacctions since bitcoin is anonymous. For users it would be clarifying in order to change bitcoin to fiat, but if the network can function by itself and make transacctions p2p I don't see what the role of states will be and what justification they can invent to tax activities.
Actually this is main reason. Governments know better what the advantages bitcoin is carrying along are but they only waiting for the time that may be in upcoming future bitcoins feature i.e. decentralized would be changed and all these governments can control the yes and no of bitcoin. But we hope a time come in near future when all this happens and we would be enjoying the perks.