There doesn't seem to be any explanation as to why or who gets the invitation!!   I seem to be one of the lucky ones ..... but I don't get the invitation very often.   Just try the link (it seems to be around from about 12 to 20th of the month.

http://sellercentre.ebay.com.au/promo/2664

This month I tried it on 14th and promotion was from  from 16th to 25th.   No invitation this month though.    Save the link and keep trying .... good luck. 🙂