As said, poor management, and too powerful unions, particularly in the 70s. Towards the mid/late 80s, they were actually producing some decent cars, mostly due to a semi partnership with Honda. It wasn't just technical, some of the Japanese work culture flowed over, and Maggie had already killed the Unions.
Come the mid 90s, disaster struck, as BMW bought them for the technology and processes, rather than to keep it as a going concern, and although it lasted another 10yrs, it did so with no investment.
I've had a bunch of BL/Rover cars, not a bad one among them:
'82 Triumph Acclaim HLS, aka YEGGY - my first car, immaculate condition, went like a rocket (for a 1.3), had the clocks reading over 120mph on my private road once. OK, too small for a family, and suspension a tad soft, but a great little motor. Killed when a Volvo took me up the arse, hard.
'84 Metro VP, aka The Thrashmobile - Mrs TB's car when I met her. Comfortable, and with the MG engine, adequate performance. Really comfortable with its VP interior. Sadly, a frequent target for the Aylesbury kids to break into. Scapped when it needed too much suspension work.
'90 216Gsi, aka TAVVY - bought cheap at 40k miles with a suspect clutch (part ex'd my Astra GTE that I HAD to get rid of quick), that thing embarrassed a lot of cars, both in acceleration, braking and cornering. 3rd gear was particularly versatile, pulling from 15mph through to about 120mph. Gave it to one of my bro's at 80k, as he was going through a hard time, he ran it to 140k without doing *anything* except tyres. That includes oil. Still went like the clappers, though acceleration needed patience, as the clutch had gotten worse

A pair of 420SLDi's, one a 96 one a 98, on her works lease scheme. Both fine cars. Not the quickest, being diesels, but satisfying to blow soot on some supposedly quicker petrols back in those days, and a million times better than the diesels Vauxhall and Ford were pushing. Good mile crunchers, and economical even when hoofing. The 98 one consistently gave 45+mpg, even when thrashed.
My little 25 1.6 iS, so spectacularly killed by the bitch with the Passat. And the Audi A5. Best handling FWD car I've ever driven, as long as it was on Pirelli P6000s. I've never known such a shit tyre to work so well. Being only 110bhp, it was showing its age later in life when it came to trying to out accelerate newer cars, but still great fun. Stunning condition, right until it lost that argument about 4yrs back. K series, so obviously it had a HG go at 72k.
So in summary, some Rovers were decent. Some just needed avoiding - Montegos, Meastros, Itals, Marinas, Allagros etc etc.