This seems to be the reason why:
The reason given for the page being deleted was "Article about a band, singer, musician, or musical ensemble, which does not indicate the importance or significance of the subject"
One of the conditions of inclusion is that something written about has to have some significance. So no, Wikipedia doesn't have "everything". In this case, the reason for deletion indicates that the article gave no information as to how this artist reaches any significance - one YouTube video does not do it (otherwise I would have a Wikipedia page).
There are numerous pages deleted from Wikipedia every day for lack of significance. Let's not start a conspiracy theory against poly where there is nothing plain. If you read the aritlce on Polyamory you will see that it gets quite a treatment. http://en.wikipedia.org/wiki/Polyamory
In conclusion, I disagree with your assertion.
Hmmm. Wikipedia appears to have a significant and relatively balanced article on the topic of polyamoury.
It also has numerous articles about personalities that an openly poly such as Tilda Swinton.
The comments on deletion about the stub for Must Be Tuesday appear to suggest that the reason was the article did not make clear the significance of the group, so as to warrant them having their own individual entry.
So it would seem you might have jumped the gun there and they are not so anti-poly.