Re: ...there's a reason for that...
I suspect the effective video bandwidth of a digital signal is much higher - in theory you could encode adjacent pixels as white and black, whereas an analogue signal was much more restricted (test card with frequency bars, anyone?).
Actually, it's quite the opposite.
Digital TV is compressed - even prior to transmission, the signal is invariably quantised in the frequency domain, so you will lose all that detail.
Analogue baseband material does not have that problem, and only loses effective bandwidth at broadcast due to noise (per Shannon's Law). Given a clear transmission channel - which was certainly possible with UK terrestrial broadcast - the displayed picture could closely match that baseband image. The SNR of a digital signal will necessarily be much lower, unless you're using a very noisy channel (poor antenna, far from source, reflection interference, you name it...)
Vic.