Because of course there is no club that better recalls Jordan Spieth's epic 2017 Open Championship win, even if he was not satisfied with his shot from Birkdale's range. It was the driving iron that took center stage and no one who watched the scene unfold will ever forget it. 
Since it's the club he used to hit the recovery setting up an epic unplayable lie-bogey and eventual win over Matt Kuchar, Spieth admirably donated it to Royal Birkdale. They received the club today, to go with their incredible clubhouse collection of memorabilia from past champions.
(Todd Lewis took us on a rare tour during this year's Open and I can concur after a tour from former club champion Ethan Davies that it's as good a display of historic clubs as any in golf.)
The question remains, does Spieth's drop warrant a plaque? I say yes, even if only the driving range attendants will be the only ones who see it!