I would imagine part of it is that our voices change gradually. Even in extreme cases, like when we go hoarse or get sick, there's an essential progression to link our new voice to the old, and there's an identifiable common thread. Meanwhile, for Hawking, it would have been a sudden, disjoint change - and with the exception of the mentioned upgraded CallText voice, any replacement would have been completely new, utterly unrelated to his prior voice; Imagine how disconcerting it would be to awake with a stranger's voice, with no expectation of getting your own back.
Exactly. And whereas biological human voices deteriorate with time, the proposed replacements for Hawking's voice were supposedly improvements (though not necessarily improvements to him).
This really hit me when I watched The Force Awakens. I was so weirded out when I realized I was expecting Han and Leia to sound the exact same as they did in Return of the Jedi. The difference in their voices is really stark. I was much better prepared for hearing Luke's voice in The Last Jedi after that.
I knew they had gotten older, I knew their voices were different, but it never hit me until I saw them playing Han and Leia again.
