After all, it shouldn't.
Partly, it matters because of the media concentration on the SWP in their treatment of opposition to the Iraq War and thus to the wider neoconservative war agenda, an opposition which embraced and embraces 90 per cent of the British population. Such are the media's backgrounds that the SWP was and is the only part that they could even begin to understand.
That suited the SWP down to the ground. Meanwhile, the mainstream opposition to that war and to those wider war agenda has been as ignored as the left-wing opposition to the EU, or the work of trade unions in promoting high-employment energy independence, or the entire tradition of rural Radicalism, to cite only three of the most obvious examples.
Bringing us to the main reason, which is the very British lack of anything to anchor the Left while engaging fully in the battle of ideas at every level of cultural life and of the education system; while refusing to consign or to confine demotic culture to "the enormous condescension of posterity"; while co-ordinating broad-based and inclusive campaigns for human rights and civil liberties, for peace (including nuclear, radiological, chemical and biological disarmament, and including against the arms trade), for environmental responsibility, and for the defence and extension of jobs, services and amenities; and while acting as a friendly critic and a critical friend of, in British terms, the Labour Party.
At best, and just imagine what the worst must be if you do not already know, we have to make do with the SWP, "Workers" who have never so much as ironed a shirt or changed a light bulb in their lives.