Hypothesis:

Often times, body parts that are not used will slowly shrink and eventually disappear (see: the human appendix). This is because creating and maintaining organs uses up body resources that could be sent elsewhere. So if a body part is no longer useful, it is evolutionarily advantageous to cease growing it.

What if pokemon "evolution" is this process in reverse? Many pokemon, when they evolve, gain significant size and mass, and all of them get an increase in physical ability. All of this stuff takes energy, and will certainly increase their body's energy requirements, meaning that they must find more food in order to survive (and are at a greater risk of starving to death). Thus, the increase in power only makes sense if they live in a dangerous enough environment to make it worth the trade-off. So pokemon are born at their lowest – and most energy efficient – form, and evolve as they are exposed to combat (i.e. "leveling up"), meaning that their bodies can automatically calibrate themselves to the environment's level of danger – a process that trainers have taken advantage of through pokemon training. It is therefore conceivable that a pokemon living in a region that is relatively safe could go its whole life without ever evolving.

Thus: evolution explains evolution.

And yet again, this reminds me of BitF.