I've looked into it, and this is a deliberate behavior from Apple.
Here is an (edited) extract of the cache for Xcode download locations:
<string>iOS 4.3 Simulator</string>
<string>$MAC_OS_X_VERSION < "10.8.0"</string>
Note the explicit "ActivationPredicate: $MAC_OS_X_VERSION < 10.8.0" setting, which is not present on other simulator packages.
I tried to edit the file, but it is replaced by another version as soon as Xcode opens. And I couldn't manage a manual download of the 4.3 Simulator DMG: the ADG website throws a "Session expired" error.
Looks like Apple had issues with the 4.3 simulator, and decided to disable it. Which sucks for people that have to support this version.http://stackoverflow.com/questions/11651773/how-to-install-simulator-sdk-4-3-to-xcode-4-4-on-mountain-lion