Because you probably accepted invites for "Windows Live", not for a specific product (thus being Messenger) of the "Windows Live" suite.
This is/was stated in those threads which explained how to sign up. But I think 99% of all people simply read "invite to beta test WLM" in their enthiousiasm (understandable), instead of reading what it is/was really about.
The same with signing up into the "beta program". It doesn't mean signing up into the "Messenger beta program". Instead, it means you put yourself on the list of available beta testers for whatever product needs testing in your regio (thus it also doesn't mean you are already selected though).
And/or as a last reason to explain why you have invites to other beta programs: It's a gesture from microsoft. They will make other beta programs available to you if you have signed up for a certain beta program (again dependable on the regio/country you're in though).
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About these
"I already got on" - "Me not" posts: The invites (and everything else) are send out in batches. These are also very local dependable, aka the country you're in. Not every country always participates in the same beta's. Or not every country participates at the same time. Or not every country has the same beta program for a specific product or undergoes the same stages of beta testing, etc...
(eg: in case of WLM, it wasn't even up till recently that the decision was made to put MSN Netherlands also in the WLM beta program. And even then, they have a slightly different beta program than others this time (closed moderated vs. moderated vs. open, etc...))
If you are invited, nice. If you don't have an invitation mail yet, simply wait and hope (posting that you don't have one yet isn't going to help you getting one faster though
).
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