I can see why you were both surprised/upset by it. Nobody likes being left out - be it deliberately or innocently.
Objectively as everyone’s saying you wouldn’t necessarily expect your fiancé’s uncle to go to a stag do - BUT in your case you’re clearly very close to them & so as always on MN context is paramount & usually competent ignored.
Especially as every man invited to the wedding was invited & you went to the trouble of going to Florida etc supporting them etc.
My only thought is & forgive me if it sounds a bit unkind - but is there a possible reason why they maybe wouldn’t have invited him?
eg does he have form for getting too drunk/rowdy etc?
What’s his relationship with the groom actually like? Are they close? Does he go on nights out with him/them typically??
O’wise it’s impossible to know why. It would have been kinder for them to say there’s a limit on numbers or given an explanation rather than just saying it was an omission.
Does this tell you you’re not as close to them (or at least your DH isn’t) as much as you thought they were?
Would make you re-evaluate the relationship a bit - if you are that close - as you say - it would be impossible to think otherwise IMO.
Maybe say you were a bit surprised /hurt & then let it go. Have they included you both in sharing photos after the event /the usual things hollowing on from the wedding? Acknowledgenent for gifts etc (if given or for your attendance to Florida etc)?