Why would a Capitals fan come in here and ask this? Are you lost gintonic?
Well, to be honest I'll give credit where credit is due, your fanbase is much more knowledgable about things leaguewide than around DC. To posit the same question there, there would be responses like - of course it's the only one no other team could think of anything that good - or stuff like that.
They've had that siren since the mid 1980s at least and I'm glad that they didn't abandon it just because horns became fashionable - so they decided to combine it with a horn but they still maintained some kind of continuity.
Back in the mid-80s the only team to use a horn was Chicago and then Buffalo joined them next.