Cannot enable "Search suggestions" in "Google Search Provider"

At the bottom of the Manage Add-Ons page you will see, "Learn more about search engine preferences" click on that.

Scroll down to, and click on "To turn search suggestions on or off" it will lay it out for you there.

Post back if you need more clarification.

Due to human nature I concentrated on your thread title, and interpreted/equated, not available as cannot enable.

You probably have a corrupted/crippled install.
If you have just the one instance of google installed, you will have to remove it, and then re-install it.

If you have problems deleting it, it maybe because it is the default.
For the time being switch to bing as the default, delete the google entry. Then do the re-install.

You may want to try using IE's version: Internet Explorer Gallery | Add ons | Google Search

For an explanation: Remove older crippled Google Search Provider
It is for IE9, but I would think the same affect would happen to IE8.

Scroll down to the second screenshot, he explains that the crippled google had a .gr suffix instead of .com that is why it was crippled.
Even if yours does have .com I would still get IE's version, sometimes the two do not play nice when they intermingle, ever notice you can only download updates from microsoft with IE? Other browsers do not work.

Your welcome.
Thank you, for letting us know your all fixed, how you went about getting it done, and marking your thread solved.

As far as your wonderment, there are too many reasons to list.
I can only offer that a higher entity may know the answer to that. (If you believe in one)