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?