It's common sense that the overwhelming majority (>99%) of racist abuse targeted at Prem footballers is from abroad.
So like has been said over and over in these sorts of threads, mandating ID checks in one country does absolutely feck all towards achieving anything. The abuse it would stop from UK residents isn't even a drop in the ocean. Meanwhile the downsides for privacy are enormous.
Even if Twitter and Instagram did a 180 and enforced some traceable ID system for all their users across the world, you would need to force India, Russia, states in the Middle East etc to pursue and prosecute these people, and they obviously won't be doing that.