For me its cheap motoring.
Got my 1st D1, a 200Tdi J plate just over 7 years ago for £2300.
Its now got 227000 miles on the clock, has been abused and maintained to within an inch of its life, done high speed (its all relative) runs from Pembs to Scotland, towed 3 ton trailers all over the country, offroad 3/4 times a week, school run duties, etc etc etc and all on veg oil (about 70K in my ownership)
It sits on standard springs with 235.85R16 Cooper STT's, winch bumper and winch, all the carpets stripped out, R380 box, and thats about it.
Recently decided to get something a bit nicer to take over the road miles and too last another 7 years, so a 1996 300Tdi Jap import was got last summer. 37K on the clock and all the toys inc full leather for £2k.
Now the 200 sits on a modified policy with Frank Pickles for £83 a year and does all the rougher stuff keeping the 300 as nice. Planning a twin tank set up to run the 300 on WVO and maybe even start running the 200 on waste engine oil.
THAT is why the D1's are more popular.