I'll take spanish for my example, but it's also true for French an Italian.
In order to remember the gender of nouns, they are almost always found with an article. It's often a definite article. I've even heard things like this :
"la rata" is a rat, but "el ratón" is a mouse
It gets in the head of the learner with the article. Why are indefinite articles rarely used? It feels much more natural to say "a cat" than "the cat" if you need too add an article.
My guess is because "la/el" and "le/la" are easier to tell apart than "un/una" and "un/une", but that wouldn't be a good enough reason if I were to teach such a language.
It doesn't even work in every case. For example in French, "l'image" is feminine and "l'étalon" is masculine. You have to say "une image" and "un étalon" to know the gender, it doesn't help at all to add a definite article.