It's because the definite article would contract and you'd not be able to differentiate la and le.

l'abeille --> From this it's impossible to deduce the gender because l' could be either la or le. Therefore if you write une, the gender is clearly feminine. Same goes for âne being masculine.

Le poulet -- the chicken (denotes the species without reference to gender)
La poule -- the hen (that's a female chicken)