I used to get that back I the 80's, people who would ask me, WHY do they keep changing girls on WHO? There is a definite difference in attitude among actors between America and England. Here, generally, if someone gets a series that becomes a hit, they tend to stay with it as long as possible. In England, it's just a job, and many actors circulate between TV, films, and, MOSTLY, stage. Stage has become so marginalized in America, it's probably hard to fathom that some people we see on TV or in films might have spent MOST of their career doing live plays and touring around England from city to city. (Before he came to Hollywood, Claude Rains was one of those.)
In the case of WHO, I suppose the trend was established the moment Carole Ann Ford got fed up with the material she was being given, and left after one year. I've been delighted to find that so many fans agree with my own feeling that her replacement, Maureen O'Brien, was a huge improvement!
At the end of the 2nd year, William Russell (who'd effectively been the show's HERO for 2 years) and Jacqueline Hill both left. As did the original producer, Verity Lambert. The show's star, William Hartnell, was very upset by this. Incoming producer John Wiles, who took over several weeks into the 2nd season, apparently didn't like or had a falling out with Maureen O'Brien, had her written out of the show. If it's so hard holding on to actors, you'd think the LAST thing you'd want would be someone in charge deciding they want to REPLACE an actor, especially when they can't even bother coming up with a character worth replacing them with. But this pattern was also established right here.
For its first 6 years, the show used to run anywhere from 39-44 weeks a year. The first real gap came between the 6th and 7th seasons. For a short time, there was a very real possibility the show would end with the 6th year, which is why they wound up finally revealing so much about The Doctor's background in the last few episodes. But when the go-ahead was given to return, they'd already made the decision to change the format completely, AND, cut the season down to only 26 weeks a year (and in fact, season 7 only ran 25 weeks). The start date for the 7th season was also pushed back several months, so for the first time, there was a LONG gap between seasons-- but still , less than a year.
The first time they did have an excessive gap was between seasons 22-23 when they show was "put on hiatus", which turned out to be 18 months. An entire season of stories, already planned, written and ready to go before cameras only days before the decision came down, was lost as a result. This was the direct result of 2 people, higher-ups in the BBC, who despite an endless series of "explanations", in truth, just PERSONALLY didn't like the show, and spent years trying to find ways to get rid of it. The BBC's most-successful program worldwide, as far as sales AND spin-off merchandise was concerned, and the guys in charge deliberately sabotaged it!
When it came back it had been cut from 26 to 14 weeks (they actually claimed the season was "longer" since season 22 had been broadcast as 13 45-min. episodes-- a taste of things to come 20 years later!). During season 23, not only was Nicola Bryant kicked off the show (on the orders of those same BBC guys), but right after the season ended, the producer was ordered to fire Colin Baker-- the show's STAR.
I still think it was completely NUTS that, after it took SO long for the show to finally be revived, and it turned out SO good, that Christoher Eccleston had a falling-out with the producer and decided to leave after only ONE year. Totally insane! His departure hit the news after only ONE episode had actually aired in England... can you imagine how that hit fans at the time? Of course, as I've been reading, nobody picked up the show in America until a YEAR later, by which point they'd already been working on the 2nd new season.
