Would be a combo of a small target-market vs the development cost; they wouldn't sell enough to make it worthwhile, and it'd end-up costing GTS money (with no possible way to get Commodore handling).
I believe Ford are doing a Ranger Lightning (baby brother to the arse-sounding F150 Lightning), but it's going to be V6 EcoBoost.
I'm not so sure the market is as small as they think it is. On the ground dealing with end customers I'd be confident in suggesting a bunch of sales straight up (myself included). Problem being that it doesn't fit into Holden's new age yuppie bullshit image they're trying so hard to gain.
As for the development cost, if an aftermarket joint can do it Holden can do it easily enough (from what I've gleaned from some of the guys I've spoken to, their engineering guys have looked into it). I don't think anyone would buy it expecting Commodore handling, I'd be happy to compromise handling off - I like the way the new Colorados drive, but can't deal with the lack of grunt and the turbo lag. Oil burners aren't really my thing either.
I really think Holden need to go out on a limb. Consider - Holden sell quite a lot of Colorados. Holden also currently sell a lot of V8's. Look at Ford with the Mustang - demand is there if you do it right, and nobody else offers a V8 in a ute except Toyota and it's diesel. Cost wouldn't be a great deal different to build either (if anything it will be cheaper as LS motors are much cheaper than common-rail turbo diesels).