Many good answers, but I'll add another angle I don't see any replies covering, which is that being poisonous/toxic is expensive. We humans lead charmed lives by the standards of the biosphere, where we get obese, and even before we got obese, many of us had unbelievable access to nutrients and energy. The steady state of the ecosystem is a war where every calorie must be spent carefully. This is particularly clear in the bacterial world but it progresses up to macroscopic plant life as well. Producing poisons is energy you could be using to grow or reproduce. Some poisons require additional care because they're still poisonous to the producer, it's just that the producer spends additional resources on containing the poison so it doesn't affect them.

There is a constant, low-level evolutionary impetus to stop spending any calorie that doesn't need to be spent, which would generally include the production of poisons of any kind. This low-level impetus is clearly something that can be overcome in many situations, but it is nevertheless always there, always the "temptation" to stop spending so much on poisons and redirect it to growth or reproduction. Over time it's a winning play quite often.