The same reason they aren't showing active subscriptions. It probably looks really, really bad.
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Because these live letters are as much about saving face with fans as providing information. SE's information releases are deliberately done in the way they are to avoid bad press, which is a good thing for the health of the company. Since they made a point out of their total number of accounts as advertising fodder, they don't want to give numbers that run counter to FFXIV:ARRs advertising campaign. Also, raid participation can be seen as a statement of the quality of the product, so it is likewise avoided.
http://i.imgur.com/nWAZAXt.jpg
LOL ...how is that my queues for frontlines are 1h+++ as maelstrom ¬¬
Out of everyone who has played: defintely less than 1% has cleared t5. Most people only play a littke bit before quitting.
Out of only level 50s. Closer to 10% I imagine.