I will chip in since I am also using a very old Jenkins version, and suspect it might be a similar reason.
My experience has shown that for our larger setup it is highly problematic to upgrade Jenkins. Every single time I upgraded I had to spend days on end trying to workaround bugs and new incompatibilities that were introduced. I ended up just freezing on 1.565.1 which gave the least problems at the time.
In short, the reason to use an old Jenkins versions is that the upgrade procedure is too costly.