Why did Igneel not attack Acnologia on Tenrou Island? For those who have read the Tartaros Arc.


   In the Tartaros Arc we learn that the Dragons were hiding within the Dragon Slayers since the year 777. Hence, why Natsu and Gajeel could not leave the guild during the battle of Fairy Tail. We learn that they were hiding to build antibodies to prevent Dragon Slayers to turn into Dragons once again and were waiting for a moment to defeat Acnologia. Yet they had their chance to defeat Acnologia on Tenrou Island. Why did they not seize their chance then and there? With Makarov, Gildartz, Laxus around, they would have had a much better chance of defeating him. So why did they not attack 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