As I mentioned on another thread some time this last week, the party heirarchy, mostly from Kinnock's winning of the leadership-onward, slowly but surely eroded the scope of the power that the constituency parties, the trade unions and the NEC could exercise over policy and over party direction generally. This accelerated under Blair to neutering constituency parties into a "rubber-stamping" role whereby they showed approval of policy or otherwise find themselves being investigated by central office, repeated attempts to not only have placemen on the NEC, but to exert influence so that it comprised entirely of placemen, and the rendering of party conference from a foundry of policy-making to a talking shop to let the membership (such that remains) blow off steam while the leadership go their own merry way.
That, sir, is "how so".