One thing that all of them —except for na— have in common is that in modern Spanish, the element after the preposition is tonic, that is, it has a stress. In the case of del and al, there are two separate unstressed elements, one of which ends in a vowel and the second of which begins with one. That heavily favors combining1 as that happens naturally in spoken Spanish with unstressed vowels anyways (tengo que escribir will probably have a normal length /e/ for most speakers in normal speech, but que eche algo will more likely feature distinct vowels via lengthening and volume change because the second is stressed).
So when the changes started happening that favored distinct vowel sounds in most combinations, in the double unstressed vowel cases (which are basically just del and al, the distinction never occurred, and although today a el wouldn't end up al in speech (probably more likely /ajl/), it remained fossilized.
1. Based on some other peninsular languages, I wouldn't be surprised if the vowel in el was epinthetical for some period. In Mirandese it's literally just l (l gato = el gato) and in Asturian and Aragonese around vowels the tendencies is to reduce it to l’– or –’l (l’asturianu, l’aragonés, diome’l llibru).