Because it's expensive to repair bugs and incomplete engineering?
Not just expensive, but the amount of times it would break down would dampen the ownership experience considerably. After all, even if I were Ralph Lauren I would still own the cars to drive (unlike him).
Apart from that, I think it is an ungainly thing, the lines are all wrong. It's just an elongated F355 with a V12 engine. I doubt the road handling will be that great, it was not a chassis test bed, just an engine test bed. So how she drives is anyone's guess but I'm sceptical.
Not the prettiest car I have ever seen. In profile, it's proportions remind me of a Zonda - too cab-forward for my liking - and the 348 was not the greatest starting point aesthetically speaking, though I guess looks were not important in the context of its original purpose.
Ignoring its appearance, I still cannot see why, rarity aside, anyone would want to have a car that was, almost be definition, not as good as it could be, i.e. as the finished product. And for me, rarity does not necessarily = desirable.