Because they'll have to implement it in .Net which is not a small task.
Even if there is an implementation in .Net it will have to be audited and even then you are at the mercy of who ever maintains it.
Sucking in OSS libraries might sound easy and smart but it's a huge gamble unless it's a defacto standard in the industry or is being maintained by a giant corporation you might be stuck up the creek without a paddle as soon as the person maintaining it decides to drop it.
