Many breeders & vendors of snakes (especially pet stores- almost always!) do underfeed snakes so they don't grow too fast & out-grow their temporary quarters- so they stay "small, cute & appealing" & because they're being "cheap" (or rather, "profit-minded") about their food.
Snakes don't put on weight quickly- please don't try to push it. First because a snake that's slightly underweight is generally healthier than one that's slightly overweight- snakes don't lose weight easily & can get "fatty liver disease". The other thing to keep in mind is that young snakes grow imperceptibly in length, so a little weight gain goes unnoticed- but they actually ARE growing- you just can't see it easily.Just be patient.