That's also pretty much the reason why Zero, as a protagonist, kind of sucks in the Zero series. He has no character, no sense of humor like he used to have in the beginning of the X series and all that.
Sorry to sort of necropost (Okay 20 days but that still puts me on edge) but I do feel like Zero did get back a little personality by Z4 at least. The most obvious being his sarcastic behavior concerning Cocapetri.
But I can agree that he did lack a lot of personality during the Zero series. MMKB does a pretty decent job of explaining why.
During the Zero series however his personality remains mostly the same although with some changes, given that he's been in stasis he is ignorant of some matters and confused about others, sometimes to the point he starts doubting or believe he can't win. This is seen a couple of times, first when Ciel was kidnapped by a Golem and he was battling him with his Buster which was not doing too much damage, was slowly starting to lose hope until X gave him his Z-Saber back or when Dr. Weil told him he was a copy, which made him believe he was not Zero, until X claimed he was indeed Zero, thus somewhat swapping roles with X in where X is the one that encourages him to fight instead of the other way around as it used to be during the X series.
I do have to agree emerging from stasis after a hundred or so odd years would kind of disorient me and somewhat cut me off from other people. Still, Zero is meant to be a tragic hero, so I think his emotionless attitude suited the darker theme of the series.
On topic though, I can't really go up against Zan or Saber's word but I always felt like by Z4 he remembered at least all the impotant aspects of his past, but never really brought them up just because, well,
I really don't think Zero cared
But I'm going off on a bit of a confused tirade here. That's just my two cents.