Through some unique circumstances, it is possible to have Pokémon at a lower level than they are usually available via evolution. Underleveled Pokémon appeared as early as Red and Green, with level 4-6 Kakuna and Metapod available in Viridian Forest. Kakuna and Metapod cannot be obtained by evolution until level 7.
Bulbapedia, Underleveled Pokemon
Honestly, there is no answer except (and I truly hate saying this...) Gamefreak wanted it that way. Because I was Googling like crazy, and there was no answer to it, except this little tidbit from Bulbapedia. And even then, it doesn't answer.
Now, just me talking here, but training underleveled Pokemon is basically the same as training pre-evolved Pokemon. Except you will get moves slower, or you will miss out on moves the pre-evolved form can only get. Their stats will pretty much be the same in the end. And underleveled Pokemon actually use up more exp points, so they kind of waste that.