Why can't I update Logic Pro X to Mac Pro 2008

Because Logic 10.4.2 requires Mac OS Sierra (10.12.x) and the Mac Pro Early 2008 (3,1) is incapable of running that OS.

The App store detects your OS version and won't let it be downloaded to a computer that cannot run it.

My understanding is your Mac Pro do not support the Mac OS version (Sierra, 10.12) that allows the latest Logic version (10.4.3) to be installed.

You want to update to which Logic version?

A 10 years old computer is in fact at risk of not being able to run the latest software on the latest OS.

I'm old enough to remember when a desktop has to be replaced in two years and a laptop was outdated in 3 months ;-).

Apple hardware is not so bad when you look at it now.

While I do not support everything Apple is doing we are not talking about 4 years of updates if I understand you correctly.

My opinion, is whatever is worth spending depends on what you're doing with your devices. I use mines for a living so the $2000 cost is not really an issue. Your case may be different.

I have very old devices for which no updates is available and they are still used for production work. It is about managing your production setup.

In the end, it is all about how you produce things whit your hardware/software setup. You have to manage it.

The MAC App Store will only show the latest version compatible with the MAC OS version you have. If Logic Pro X 10.4.2 doesn't appear in the updates it means you can update to an earlier only.

I suppose it's a hint that you need a new computer ;)

Well, I didn't mean "brand new" computer. You might be able to find a more recent model of used Mac Pro or MacBook Pro below $2,000.