score:2
I get the impression that a "human" birth is seen as the best for gaining enlightenment:
The formless realms are associated with the highest heavens -- see here for example: The Thirty-one Planes of Existence
Combining 1. and 2., perhaps it stands to reason then, that if the formless realm results in highest heaven, then it is not conducive to enlightenment.
That's maybe in addition to more mechanical explanation, like,
The inhabitants of these realms are possessed entirely of mind. Having no physical body, they are unable to hear Dhamma teachings.
Also earlier in MN 26 you have it that Alara Kalama himself attained the formless realm, and who taught the Buddha when he was a seeker -- and was (unlike the Buddha) apparently satisfied with that.