This fact, observed by a reviewer at Outpost Gallifrey, prompted me to echo his basic question: just when did season finales have to be bigger and more "epic" than other episodes in the season?
In American television, this has indeed been the pattern for quite some time, especially in the "cliffhanger" phenomenon, which I have comments on, but will leave those for another time. In Doctor Who, it's not quite as common. Looking back at the old series, I'd say the following seasons ended with "special events" (admitting that the term is being rather loosely and unscientifically defined at the moment, and acknowledging that classic-era Doctor Who is best understood in terms of story arcs, rather than as individual episodes):
- Season One: "The Reign of Terror" (first example of the Doctor playing a double role, this time as one of the antagonists)
- Season Two: "The Time Meddler" (the first appearance of another member of the Doctor's own race)
- Season Four: "The Evil of the Daleks" (intended to write the Daleks, the main villains of the franchise, out of Doctor Who entirely. In fact, they appeared again just over four years later)
- Season Six: "The War Games" (a 10-part story designed to write Patrick Troughton out of the role of the Doctor. Featured the first appearance of the Doctor's still-unnamed home planet)
- Season Eleven: "Planet of the Spiders" (designed to write Jon Pertwee out of the role of the Doctor, and showing the regeneration into Tom Baker. Other Pertwee-era season enders did feature the final appearance of certain companions, but I couldn't otherwise justify including them in this list)
- Season Twelve: "Revenge of the Cybermen" (the first appearance of the Cybermen [not including a cameo in "Carnival of Monsters"] in six years)
- Season Fifteen: "The Invasion of Time" (takes place on the Doctor's home planet of Gallifrey. However, it should be noted that "The Deadly Assassin," which was the first ever story to be fully set on Gallifrey, was not a season ender.)
- Season Sixteen: "The Armageddon Factor" (as the final part of a year-long story arc, of course it was special!)
- Season Eighteen: "Logopolis" (the departure of Tom Baker as the longest-serving Doctor to date)
- Season Twenty-One: "The Twin Dilemma" (the first story featuring Colin Brown as the Doctor. This is the first time that the final story for an actor as the Doctor did not take place at the end of a season since William Hartnell, the First Doctor)
- Season Twenty-Three: "The Ultimate Foe" (also known as the final segment of the year-long "Trial of a Time Lord" story. See Season Sixteen for justification. This also happened to be the last story to feature Colin Brown as the Doctor, but since this was not intentional, his last appearance hardly counts as justification for calling the season finale as an "event.")
- Season Twenty-Four: "Dragonfire" (the appearance of Season Twenty-Three character Sabolom Glitz, the first appearance of companion Ace, and the departure of companion Mel, taken together, seem justification for calling this an event, but any one of these in isolation probably wouldn't)
- "The Three Doctors" was the first story of Season Ten (almost a full year before the actual 10th anniversary of the program).
- "The Five Doctors" didn't take place within a conventional season at all, but aired around the 20th anniversary of the program, between seasons Twenty and Twenty-One.
- "The Two Doctors" was smack-dab in the middle of Season Twenty-Two.