Why did Igneel not attack Acnologia on Tenrou Island?
Because the dragons inside dragon slayer's bodies were waiting for The Dragon King festival as that would be the time for them to take action and by then enough antibodies would have generated to prevent Dragon Slayers form turning into dragons

Source : Fairy Tail Manga Chapter 400 Pages 12-18 and
Tartaros arc summary in http://fairytail.wikia.com/wiki/Igneel#Synopsis