Only because of a $3.5B write off that is carried forward with accounting. JLR is still consolidating while Tata is seeking partners.
The Defender was "old" and not invested in because they were part of the now defunct Rover Group. LR barely survived. The D5 might be another 'best product' that sold ..... 450 in the US last month while Jeep sold 24K.
Seriously, you can't have it both ways. Either you take advantage of technology - including suspension, brake materials, and so on (not just computer screens), or you get left behind in capabilities. The LR Defender was left behind decades ago due to to advances in technology (materials and computers controlling such things as traction control etc.).
The Defender wasn't sold in that time period you talk about, and you're only thinking of the US.
Jeep sold the Wrangler to both the "rock crawling" fraternity (both actual ones and wannabes) as well as to the hairdressers. It's a cheap car, built cheaply, and is cheap in quality. So of course, a cheaper car is sold more.
However, look in Europe. There's a reason there is not many Wranglers around. It's simply too cheap, too unsafe, and not that capable when it comes down to it. But of course, you're not allowed to swap axles and throw a 20" lift on it, nor are you allowed to drive around with your tyres outside of the car. So there's that.
They
did invest in making the Defender the most capable offroader they can produce (again). No, there is only so much you do with old tech.
Let me mention the Wrangler again: It is the lowest common denominator, loved by "bros" and hairdressers alike. It is a low budget, low tech, easily modified by rednecks and the like, and easily "prettied up" by the barbies of this world (no, not a hint at the doll barbie, other than metaphorically).
Why didn't jeep invest in making it up to date? Because people still buy it for two reasons: Image and the fact that you can modify it with a welder and huge tires. And if you have a little more money, you can overcome one of the many flaws of the Wrangler: Cheap and not particularly strong axles and throw a Dana axle in there.
Lowest. Common. Denominator.
Again: How much does the Wrangler tow? How much does it carry?