TV stations simply haven't got the money to pay for drama series,etc.
ITV, for example, has seen its income fall by over 90% since its heyday. That's partly because there are now far more channels competing for a finite amount of advertising revenue but even more so because advertisers are turning away from expensive TV advertising and using alternatives such as social media instead.
Premium costume dramas (with big casts and possibly lots of special effects) cost around £1 million pounds per hour to make. Most other drama series cost, say, £500,000 per hour.
Scripted comedy might cost £400,000 per hour to produce.
Daytime quizzes might only £25,000 per hour.
So-called 'reality TV' might cost £50,000 per hour.
The figure above are based upon what the BBC was paying for programme content six years ago:
http://downloads.bbc.co.uk/commissioning/site/tariff_prices_for_independents.pdf
Channel 5 can't afford most of those prices, so that simply look for whatever's left within their budget - which isn't very much!