When I was growing up we had 2 channels, BBC and ITV. Then they added BBC2 so we had three.
But these channels did not run all day, we had no breakfast TV for example, there was not much on during the day, and channels often shut down at midnight or just after.
So they did not need repeats to fill the air waves, as there was not much time to fill.
Nowadays I have Sky, and I could not tell you how many channels I have access to but it is probably 300 or 400.
And many of these channels run for 24 hours so there is a LOT of channels to fill, and a LOT of time to fill, so there is bound to be lots of repeats.
Also making TV programs is not cheap, so once you have spent a fair bit of money making a TV show, like say Downton Abbey, you are going to show it plenty of times to get your moneys worth.
And of course because "proper" TV shows are now expensive to make
with script writers, actors, film crews etc. it is easier for TV companies to make "reality" TV shows where you stick a camera in front of a group of people (hotel staff, hospital staff, rubbish collectors etc) and just film everything they do then edit it down to a 30 minute show.
No script writers, no actors, so cheap to make.
Which is why the overall quality of current TV is so low (apart from a few quality show that are made, mainly by the BBC).