Pokemon ranger is not in the steam siege set. Or in the whole expanded format. This card is important in mill and control decks and is a useful counter in some decks. Not having pokemon ranger in the game can affect how games turn out and I ask to please add it.
they haven’t added it because it’s too hard to implement
As pointed out, this is arguably the most difficult and most important card we need to implement. Frankly, this is an extremely difficult one to implement without some serious work on our backend for how we handle effects on the engine. This may happen at some point, but this is currently low priority. You are free to give a crack at it yourself if you wish to help with development.