You're absolutely right; these days it doesn't seem uncommon to see more than one finishing move (per opponent) performed in a main-event (or "major" title) match. It seems that this is a fairly recent phenomenon, but it must go back at least 15 years or so (that's when I started watching wrestling again). Before that, any finishing move meant the opponent was DONE, and not getting back up - no matter what.
You really have to understand that the story-telling in the ring now is different than it used to be. A lot of guys have more than one finisher these days (ie: Cena - with the STF, "You-Can't-See-Me" and the AA, Mysterio - the 619 and body-splash off the top rope, etc). With so many ways for an opponent to win a match, the fans almost expect to see every finisher performed (especially in a main-event setting).
IMHO, it's really weird (for lack of a better term) when a wrestler changes their finishing maneuver, or when a "signature move" becomes a finisher. There are two PERFECT examples (of the latter) that occur during the 1998 WWE (WWF) King of the Ring PPV. The first happens during a tag match (New Age Outlaws vs. The New Midnight Express), and the second happens during one of the KotR qualifying matches featuring The Rock (I can't remember which specifically, he was in two matches that night).
During the tag match, Billy Gunn hit a move that later became known as the "Fameasser". This was before it was Gunn's finisher (I don't know what move he used before the Fameasser), and I'm not even sure that Gunn went for a cover after the move. If he did, it sure as hell didn't finish the match. The weird thing is that only a few months later, that exact same move would have rendered his opponent unconscious.
Something similar happened during one of the Rock's KotR qualifying matches. I'm not sure which, like I mentioned earlier...hell, he could have done this in BOTH matches. Rock's first match of the PPV was against Dan Severn (in which Rock was victorious), the second was the final match of the KotR contest - where Rock was defeated by Ken Shamrock.
The Rock performed a move that was awesome to see, something that he must have done before this PPV - since J.R. knew enough to call the move an "artistic elbow". Yes, it was the move that later became known as "The People's Elbow" (aka "The Corporate Elbow"). All the basic elements of the move were there: bouncing off one rope, jumping over his opponent, bouncing off the other rope, then a slight pause before dropping the elbow. The only thing that was missing was the classic spinebuster, then pose, then the removal of the elbow pad (and the toss of said pad into the crowd). This move did NOT defeat Rock's opponent that night, even though the EXACT SAME move was later able to completely incapacitate an opponent for the 1-2-3 (just like the Fameasser, as I mentioned earlier).
With this onslaught of new "signature" moves becoming more and more popular with the fans, the wrestlers would be stupid to NOT perform them in EVERY match. Otherwise, the fans would leave the arena saying "...Rocky did the Rock-Bottom tonight, but why didn't we see the People's Elbow? I'm pissed...". Pissed off fans equal lower ticket sales, lower merch sales, etc. It must have been Vince, the trainers, the writers, or the wrestlers themselves that took these moves and elevated their status from "signature move" to "finishing move". Having more than one finishing move is quite common today, even though there are still plenty of wrestlers that have only one move that get them the win.
The fans expect to see all of these finishing moves performed (at least in high-status matches), and it's impossible to see them performed without seeing at least ONE kick-out. There's also the factor that others have already mentioned (so I won't get into it), but a kick-out from a finishing move also makes a match more exciting (and less predictable). There have already been so many good points in this thread about this; I'd only be repeating others if I spoke on it now.
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This next part is a little off-topic, but I wanted to mention a conversation that I had with a friend in the late 1990s about the DDT. The DDT is a move that used to ONLY be a finishing move. Jake the Snake couldn't have been the first man to perform the move, but he's the first guy I remember seeing do it (and make it a finisher). After you saw Jake hit that move, you knew his opponent was out-cold (and thought he possibly had a concussion). It just seemed that there was NO WAY that Jake's opponent was getting up from that DDT.
I don't know exactly when the DDT became a common move that would NOT put an opponent down for good (a good example of this is Randy Orton's DDT off the second rope...yes, it IS a "signature move", however it usually doesn't end the match), as I stopped watching wrestling again until the mid-nineties. When I started watching again, the only guy that could finish an opponent off with a DDT regularly was Raven (yes, Sting performed the Scorpion Death Drop as a finisher, but it wasn't the "classic" DDT). It seemed that everyone could pop right back up from a DDT, unless it was performed by either Raven or Jake the Snake.
Inversely, Kevin Nash was the first guy I remember turning the Powerbomb into a finishing maneuver. Before that, it was a common move, performed by many, that would usually not finish a match.
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We all know that pro-wrestling has predetermined outcomes. In some cases, the entire match is predetermined, move-for-move. Why do we still watch, if we know that it's "fake"? I hear that question from friends & family all the time. The answer is that most "good" entertainment is scripted (that even includes SOME "reality TV"). You wouldn't say "that's fake" to somebody if they were going to see a play written by Shakespeare, would you? Of course not. You have to suspend disbelief, and buy into a certain amount of the kayfabe if you really want to enjoy the experience. Same goes for guys kicking out of finishers; if you don't believe "the other guy" has a fighting chance, it makes for a far less exciting match. Sorry for such a long post, hope I didn't bore you to death.