It’s not a matter of the plants making a moral choice, it’s a result of too much rain. Scientists studying carnivorous plants in Swedish bogs found that the extra nitrogen deposited in the soil from rain reduced how many insects the plants trapped. Pollution from cars and power plants causes nitrogen-rich rain, so more reaches the ground in some areas.
In other words, predator plants may cut back on flies if they can access key nutrients elsewhere.
In BBC News, Matt Bardo quotes biologist Jonathan Millett as saying, "If there’s plenty of nitrogen available to their roots, they don’t eat as much. Basically, it’s like adding more fertilizer."