This is an "incomplete" cleft sentence, 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?
He climbs because a mountain is there.
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...".
He climbs, because...
So, after coming to this era, you spent three years to transform from the egg form to that form, and that's because...?