If you're looking for a scientific answer, there isn't one that I know of. Maybe because it's so much a coward it doesn't even know how to fight if it wanted to? IDK.
But this is a mechanic Nintendo has practiced many times before. Many Bug-type which can evolve and a few Water-type Pokemon are limited in their movesets. The Dustox/Beautifly, Beedrill, and Butterfree early evolution lines are the best examples of this - all weak and a bit tough to train up. Bright side for these guys is that they evolve at very low levels. In contrast, Wimpod evolves into Golisopod at level 30, though this is easy once you find the ones at Poni Island (~Lv 40).
Some other examples of Pokemon that suck and are hard to evolve due to a limited learnset include Magikarp, Feebas (though really that one's not by training), Kricketot, and Tynamo.
Why both at Lvl 1? Well, it needed SOMETHING to start with, right?