Why exactly is it only macbook 2008 or later that can run Mavericks?

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In the case of your Mac, there are different reasons:

• Your Mac has got a 32-bit EFI, so it cannot start up a 64-bit operating system.
• Intel GMA GPUs are no longer supported and drivers for that GPU were dropped in OS X Mountain Lion.

Because Apple has dropped support for those models of Macbooks. You can, of course, try to install it if you wish but I advise you to have a good backup because you will probably end of having to restore from it.

Here are the requirements:

http://www.apple.com/osx/specs/

When Apple introduces a new OS, they have to impose a limit of how far back they can support the new OS with the hardware; older models can't sustain the way the new OS operates. Both the processor and logic board (graphics card) have to be able to run it. Even if your machine could run it, it would be most likely not the best experience. The new systems devour RAM; 4 GB is generally recommended for basic computing, 8 GB or more is better especially if you do any photo or video editing. Snow Leopard runs great on 3 GB - Mountain Lion or Mavericks wouldn't like it too much.

As for installing if you tried: you will get a message that the OS cannot be installed on this computer and the installer would simply quit.