Firstly it works both ways, as JWs don't accept others are Christians either. But each for different reasons.
Other Christians generally do not recognise JWs as Christians because they don't accept the Nicene creed, which many perceive to mark the boundary of what can and cannot be considered Christian. It might seem a narrow definition, but admittedly it is broad enough to include hundreds of denominations all over the world and the vast majority of those who self-identity as Christian.
JWs on the other hand reject others as Christians not only because of doctrine but also because those outside the organisation do not recognise the unique authority of the religious leaders of their own church. So even if another group held all the same doctrines as JWs, but rejected the Governing Body, it is doubtful whether JWs would recognise them as Christian. In this respect they resemble the Catholic Church, although even the Catholic Church has shown some interest in ecumenism.