Two reasons I can think of:

1. The Ueberklass wanted direct access to meetings of ministers without the inconvenience of having to disclose what was going on.
   
2. The EU wanted the bureaucratic version of a BS grinder, a machine that looks and sounds impressive but can't actually do anything.

#2 is what we're seeing, although I'd be shocked to learn #1 didn't happen prior to the Eurogroup setting out.  The Greeks were expecting an informal body; they got the Trans Europa Fail Train.  I consider that alone preclusive evidence the EU and the troika never intended good faith negotiations.