I read an opinion piece on this topic on Ynet and have also found information about this halachic topic referenced as being in R. Shemuel Balmas's Mishpat Gerim Vol. 1, Ch. 3 Sec. 3, p. 168.
From what I've gathered from there, this is because it will be very difficult for a deaf person to fulfill the mitzvot. There's no way he can learn Torah and understand the complexity of the Torah and verbally accept the yoke of the mitzvot. In particular, the prayers that require pronunciation with the mouth.
The curious thing here is that in modern times, there are hearing-impaired people who can speak and disabled people who cannot speak at all. The reason is that hearing aids and cochlear implant surgery exist due to advances in medicine.
So, unlike the halachic opinion cited in Mishpat Gerim (p. 168) mentioned earlier, does this also apply to hearing-impaired people who can read and pronounce the Torah directly?
Or, in the case of such disabled people, is conversion not unconditionally impossible, but rather dependent on the judgment of the beit din?