Because for the amount of power needed they would have to halve its payload capacity if they used RTGs

Who has nuclear diamond batteries? Those are a total myth drummed up to get investor money. They don’t actually exist.

Sure the concept exists, in the form of betavoltaic batteries, those have been around for decades. They are tough and last dozens of years if not longer. The only problem is, they put out microwatts. You can use them in very niche applications, but those are few and far between. It’s hard to convey how little power a microwatt is, it’s basically nothing.

What you care about when going to space is energy density, and the proposed energy density of nuclear diamond batteries is very poor.

Rtgs are very useful for longterm missions, but are crazy expensive. They also aren’t being made anymore, so getting a hold of one is hard. The weight is an issue as well, they are super heavy.

These commercial moon missions are primarily demonstrator missions. They aren’t meant to last, they don’t really have a goal and often don’t do something useful. The idea is to show you can do it, so you can sell a product. Other people that do want to do useful stuff can then pay to get their stuff to the moon. So if the mission is over when the sun sets in two weeks time, that’s perfectly fine.