Is there a specific reason there are only three Treaty agendas?

By AsaTJ, in 2. AGoT Rules Discussion

It seems odd that there is no way, for example to run a Lannister/Baratheon treaty deck. Is this something they just haven't gotten around to yet, or is there a rules reason they don't want to allow this?

AsaTJ said:

It seems odd that there is no way, for example to run a Lannister/Baratheon treaty deck.
this card

Because I was not aware of it. Now I am. Thank you kindly.