Yes, because it didn't need it - the Start Menu search (for me at least) was mainly used as a quick program launcher or familiar file launcher. Start typing the first few characters of the app/document, <Enter> - boom, app launched, or drag the document that you need opened into a new window (or right-click and delete it, copy it, get properties, etc). Now through Win8:
Windows key - launches Metro, a completely alien GUI compared to the one you were just in.
With the mouse, select Apps or Files.
Enter in your search text.
Select the app/file, hit return. Or, you wanted to drag and drop? Pfft.
Yes, you can use the search field in Explorer - after you open up the Window, mouse the mouse pointer or hit Tab several times, type in your query.
Many more steps to replicate the functionality of a hotkey - which now launches a completely distinct GUI. Wonderful.
Can't you just start typing in Metro and it searches immediately. Why do you need to select Apps or Files?