Feodum has clear text below the line separating its primary use from its secondary/reactionary use. Why isn't it a victory-reaction card for better use with courier, falconer, etc.?
The designer of Dominion weighed in on this, as quoted in the DominionStrategy wiki article on Reaction cards:
The argument for not making when-trashed cards reactions - and not making when-gain cards reactions too, or giving them their own color - is that the card is involved. You know. You play Militia; Moat isn't involved, but I could use it. You play Venture; Tunnel and Patron are just going by, but they matter. You buy or trash Silk Merchant; it's right there, in on the action. So we don't need to call attention to it as much as we do Moat / Patron / etc.
-Donald X. Vaccarino
So in other words, the Reaction type was put on cards where the player is able to choose, within the trigger, whether or not to reveal and activate the Reaction. Once the player chooses to trash Feodum, they don't get to choose whether to gain Silvers, but if a player chooses to discard a Tunnel to a Militia they have the choice of whether to reveal it to gain a Gold.
While I don't have a direct quote handy, Donald has definitely discussed versions of "Why don't you make this design choice to make some other interaction work?" and he tends to prefer focusing on design choices that make that card better. So "add another type to the card so it works better with Courtier"1 hasn't been a strong driver for him.