Old sitcoms.... why did they use film for location shoots?

The same went for news, until the early 80's all (non-live) news actuality was shot on film. It had to be brought back, developed before being edited and broadcast, the turn around time for a piece of footage was sometimes days rather than minutes. In the early 80's all the TV news operations started moving over to 'Electronic News Gathering' (ENG) using tape formats like Umatic and Betacam

The 70's version of the Beeb's 'Survivors' started off as Studio VT/Film Exteriors, as the characters were on the move more in the earlier episodes but once they settled down at one location, the OB unit took over from the Film unit.

(However in a Season 2 two parter 'Lights Of London', one of the then regular characters, who was a medical student (& by default within the storyline, the 'doctor'!) was kidnapped & taken to London. As there was loads of action shots in the story, the Film unit came back for these 2 episodes!)

The Tube did a skit outside Newcastle railway station which showed the difference (at that time) between film and video from a portable Betacam camera. The video did look very washed-out and artificial next to the film.

The Only Fools and Horses episode As One Door Closes used video for location scenes, I was watching it the other and quality wise it ain't great.

U Matic was only allowed (within ITV and C4) for news acquisition [1] The IBA Code Of Practice did not allow its use for non news applications. Electronic cameras on location had to have their output recorded by Quadaplex (2
inch VT) or C or B Format, (1 inch VT). Although some programmes such as ATV's Timeslip, and the BBC coverage of the RI Lectures used an OB truck producing the footage as a multicamera shoot, but sending the footage back via microwave link to the studio centre for recording there, rather than attempting to bring the VTR to site.

[1] Almost certainly the BBC applied the same standards. The IBA did apparently allow Channel TV to use U-matic to timeshift their showing of Crossroads in the 1980s.

was this for technical standards reasons or a crazy old union rule?

Not quite the same at the BBC - the BBC "Film Unit" certainly had HiBand Umatic recorders and 3-tube lightweight cameras for some acquisition in the 80s (the bizarre duopoly of Film unit video vs BBC OBs single camera operations was part of the reason), as did BBC Outside Broadcasts. UMatic HiBand stuff would usually be edited to 1" (C-format usually at the BBC) - and the BBC "Film Unit" also had some early (non-SP) Betacam stuff (the first real camcorders)

The workhorse of BBC OBs PSC (Portable Single Camera) operations was - by the mid-80s - the Ampex VPR5 (which took small 1" spools and looked like a Nagra sound recorder with 1" rather than 1/4" tape - and a Sony BVP-330AP - where UMatic wasn't deemed acceptable. (The VPR 20 was also used)

There wasn't a straight News/Non-News demarcation as such in the BBC - and whilst UMatic was frowned-on, it was certainly used for non-News shows. (News were probably the only people who did 2m/c UMatic edits though)

And of course loads of us remember watching Treasure Hunt - where Aneka's location stuff was recorded on a very visible UMatic recorder!

Oh my goodness, yes, I'd forgotten about that !!!

To answer Pete's question, the use of UMatic was banned by the IBA for certain applications purely for technical reasons.

16mm/Super 16 film is still used quite often, for example Merlin, Primeval, The Walking Dead, The Middle and many more examples.

Over the last 15 years or so, 35mm was used very often in American television for the big primetime drama and comedy series, but with the advent of good quality HD video, saving many thousands of dollars an episode, it's in decline. There are still a few series shooting on 35mm (Two and a Half Men, House, True Blood, Mad Men) but it was said that Lost and 24 coming to an end last year was something of an end of an era when it comes to big network shows shooting on 35mm.

To reply to this and other points:
As has been described all steadicam is, is a camera mount that came into being during the 80's - it can be used on any camera.
I've worked on all film and film inserts to VT shows and would prefer one format throughout. Historically though the early 3 tube video cameras were too bulky for exterior use without a huge scanner and lighting rig - film was far more compact. I've worked on a drama where multi camera film was used but it's unusual - unless you're doing a big stunt or such like. In our case the main unit was running out of time because of weather delays but the 2nd Unit was free and as the director had confidence in his cast a couple of long dialogue scenes were shot this way. It halved his schedule for this scence at least.
As for telecine transfers - "it was fine leaving us" as they say but in my example it's technology nearly 30 years old. I expect some domestic sets are getting close to edit suite monitors these days and show up imperfections. The other thing that can happen is the transmission prints are handed over to a 3rd party for DVD making and they don't appear to make any effort to clean them before putting them on the telecine machine. That's particularly annoying after months or even years of careful work.

I know 35mm was used in the UK for the ATV/ITC series of the 60s and 70s and the quality of the photography is still excellent. Thunderbirds was shot on 35mm and still looks great (apart from the strings being more visible )

Sorry if I went off the subject a bit...I can remember seeing an episode of Bless This House a few years ago (which had to be from around 1973-74) where the outdoor scenes were shot on videotape and like the Only Fools and Horses episode mentioned earlier, the quality doesn't look brilliant nowadays. I think Thames were one of the first (if not the first) ITV companies to use videotape for outside shooting.

On a related note, I see Civilisation has been remastered from the original 35mm film and is to be shown in HD.

I take it that even the best quality 16mm doesn't stand up to this sort of upscaling?