Because the ALARM has already gotten itself into attack position. The real opportunity that you had to stop it was while it was on its way there. Once it's hanging on that chute, it's going to attack within a couple of seconds (detach from chute and guide to target), while you need probably 15-30 sec to detect, classify, realize it's not a spurious contact (that's assuming your radar is looking that high, and in a non-TBM application it may well not be), and attack - by which time that ALARM is probably already sitting in the Rmin for your system, if not blowing up one of your radars.

With SEAD/DEAD, timing is everything.