It's probably because the MD5 encryption by large is considered broken. There's no reason to support an encryption which with relative ease can be decrypted.
Dude, just change the framework to one that will suit you better if working this out is a problem big enough for you to offload here...
http://security.stackexchange.com/questions/52461/how-weak-is-md5-as-a-password-hashing-function
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