Founder effect, to a limited extent. The people who developed Lemmy started lemmy.ml, a server (aka instance) specifically for "leftist privacy and FOSS enthusiasts". This was the description that they chose for the server and a significant portion of the people who joined the server did so because of that description. Leftist is a political term.
By design, Lemmy can be pointed at different servers for different content. You can pull up https://join-lemmy.org/instances for some 'approved' servers. A given server could be right-leaning, centrist, or totally apolitical. It may be that you chose to join lemmy.ml because it seems to be the most active lemmy instance. It currently is, but it also happens to have a lot of users who are interested in politics.
Ultimately the answer to your question is "Because you linked up your app with a lemmy server that chooses to focus on politics."