I think one important reason is "marketing strategy". Most JWs (and fundamentalists in general) have not chosen to become JWs (etc.) simply because they were after an understanding, loving, compassionate environment. This they could find at a cheaper personal cost in mainstream churches, or other religious or secular communities. The appeal product was not "love" or "mercy" but "truth," life-saving truth. Harsh requirements were part of the deal -- in a sense, they made the whole thing look and sound serious (after the analogy of the "narrow way").
WT leaders can easily identify stages of organisational growth with periods of strict doctrine and policy, and stages of stagnation or decline with more lenient policy and "loving" doctrine. The second part of the 70s come to mind. The reaffirmation of harsh policy in the 80s coincided with renewed organisational growth, much like the shift from "character" to "activity" and a growing list of prohibitions in the Rutherford era. They have learnt that lesson: it would take a tremendous amount of inside pressure to force them into leniency again, because they know the long run cost would be higher. Only when the "carrot and stick" do not work anymore with the insiders do they have to cut them some slack. But they have everything to lose in doing so because a "loving" religion is simply not their part of the market.
Sometimes the "easy way" is the most difficult.