Internet and social media has made it easy for right wing organizations to spread thier propaganda and gain traction.
However, personally i believe that this right wing has grown mainly because of the failure of left organizations and the vacuum left by them in the preceding decades. The RSS and its sister organizations work in villages and small towns, they work with the youth there who are often unemployed or not satisfactorily employed, they give these youth a sense of purpose through their ideology and training. They organise blood donation drives, help in organisation of local festivals and events, they hold rallies. Basically they do groundwork and build cadre.
The left front parties have honestly failed there. We look at the north indian hindi belt states with some disregard because they are hotbeds of right wing hindutva. I think these states should give the left some food for thought and introspection. How does a region that comprises mainly of poor working classes and bahujan folks become so aggressively right wing? Where have the left failed?
To be fair though, the left is showing some introspection in this regard and working to rectify this. I know for a fact that the Left front parties are making concerted efforts to regain ground in Bihar and they have shown marked improvement in this election. Ambedkarite movement is gaining traction on UP under Chandrashekhar Azad. Muslims are increasingly looking towards AIMIM rather than middlemen organisations like the Congress. While there are concerns of communalisation of politics there, keep in mind that the political field has already been communalised and it is only natural and sensible for Muslims to look for representatives that are from their community and will work for their benefit.
There are glimmers of hope. But there is a long way to go to regain lost ground