What is the function of のは in this question?


きさまがこの時代に来てタマゴからそうなるまで3年間かかったのは...?




This is an "incomplete" [cleft
sentence](https://japanese.stackexchange.com/a/19208/5010), and that のは is
part of the grammar of cleft sentences. Do you understand the following
simpler example of cleft sentence used to emphasize the reason?

> そこに山があるから、彼は山に登る。  
>  He climbs because a mountain is there.
>
> 彼が山に登る **のは** 、そこに山があるからだ。  
>  It is because a mountain is there that he climbs.

And the sentence in question is an incomplete version of this grammar. The
speaker left the latter half of the sentence (the reason part) unsaid,
probably because he is expecting the other party will continue his sentence
and explain the matter. In other words, it's like ending an English sentence
with "because...".

> 彼が山に登る **のは** …  
>  He climbs, _because_...
>
> きさまがこの時代に来てタマゴからそうなるまで3年間かかった **のは**...?  
>  So, after coming to this era, you spent three years to transform from the
> egg form to that form, _and that's because_...?

