(This is not Shingrix specific) Before doing the large (Phase III) clinical trial that leads to approval, smaller trials in humans (Phase II) are done to test different doses and see how the body responds.
It is also a matter of risk management: assuming that two doses always create stronger immunity than one dose, it's better to do a successful Phase 3 trial with two doses and "miss out" on getting approval for a one-dose course, rather than aim for a one-shot vaccine, and risk the response being too small to get any approval at all.
Also, if the drug is successful in the market, Glaxo can do another trial to show that 1 dose is not inferior to 2 doses, and get a patent extension for the new formulation.
Why is this pertinent to hacker news?
They might have had data from lower animals that suggested single dosing would not be (entirely) effective. That or there was a need for certain blood levels to generate a response.
There’s a whole lot of testing done for various reasons before doing anything in humans. And yeah, this is one of the many, many inappropriate stories on HN as it tries to morph into a less technical, more oppositional forum.