Because Labour can be joined online at very low cost, participation in Labour’s £3 affiliate scheme has all of the hallmarks of a “clicktivism” activity. Even full membership, for the purposes of voting only, has most of those attributes for people who are not on a low income.
Labour’s problem, in a nutshell, is this.
The internet has sucked people who treat politics as a recreational tribal activity into the party, and into online political activity. These clicktivists will always outnumber and outvote the people who have joined the party because they treat politics as a means of changing society. Real world political activity is less attractive. It involves legwork, time, a willingness to think, to compromise and to make the kind of difficult moral choices no self-aggrandising clicktivist would ever consider. The anonymous variety of online activist won’t even suffer any social consequences resulting from their wrongness.
There will always be more clicktivists than declared activists. Until the party has a reason to deal with this problem, it will keep dying.