Because the Grenadier is a vanity project of a billionaire that will sell 25k units a year if he's lucky, and the last several iterations of the Defender were selling in the low thousands of units a year. Because Halo models like the G-wagen are the domain of companies with big cash reserves that can afford an overpriced marketing stunt. Because Tata is a public company that has obligations to the shareholders that Ineos simply does not have.Why didn't LR build the Grenadier?
And because a lot more people will buy the Defender than will buy the Grenadier. JLR had 22k orders on the books before the first defender was delivered to a customer. This was considered disappointing, and parroted by haters on the internet as evidence that the Defender was an utter failure. Ineos plans to produce 25k Grenadiers total per year.
The Grenadier will be cool. I might even buy one. But it's a gimmick selling nostalgia, not a car that a company that is in a financial tight spot will build its future on.