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:
~/Library/Caches/com.apple.dt.Xcode/Downloads/eded78df8bfabaf6560841d10cf8e53766f74f28.dvtdownloadableindex
<dict>
<key>source</key>
<string>
http://adcdownload.apple.com/ios/ios_simulator__resigned/ios_43_simulator.dmg</string>
<key>identifier</key>
<string>Xcode.SDK.iPhoneSimulator.4.3</string>
<key>name</key>
<string>iOS 4.3 Simulator</string>
<key>userInfo</key>
<dict>
<key>InstalledIfAllPathsArePresent</key>
<array>
<string>$(DEVELOPER)/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk</string>
</array>
<key>ActivationPredicate</key>
<string>$MAC_OS_X_VERSION < "10.8.0"</string>
</dict>
</dict>
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也就说,山狮系统中无法使用4.3模拟器..楼上翻译的还行.大概意思都出来了.