First, it took in a lot of people from banned subreddits (/r/coontown, fatpeoplehate, etc.) Unlike the great Digg 4.0 to Reddit migration, they didn't a huge chunk of Reddit. They just got the most controversial.
Second, It tried to be a full Reddit clone with tons of subverses. Reddit started out with just a few categories and slowly expanded out to user generated subreddits. That has to grow slowly over time with your userbase. Since Voat started with that, it has a ton of abandoned subverses that one one posts in anymore.
Voat is a pretty bad cespool of hatred right now. There's occasionally good stuff on there, but a lot of it is conservative garbage. People who have tried to turn it around are so outnumbered that they just leave.
With that said, Voat is open source (written in C# I believe), just like Reddit.
http://slatestarcodex.com/2017/05/01/neutral-vs-conservative...
(see in particular section III of the article)
'The whole thing is based on voting which is why it has "vote" in it. It has nothing to do with voat.co. I guarantee that'
EDIT: The article implies a connection, but as cracell pointed out, there is in fact no association, even historical or in personnel.