I think Ishant just doesn't really seem to learn. When he first came in, he was bowling in the 140s, and I recall on the Australian tour he got one up as high as 152. He is more in the 130-135 mark now, but that means that many of the deliveries he may have got away with before won't work. His bouncers just seem to sit up and beg to be smashed, and he dishes up way too many half volleys and length balls in the shorter forms of the game. I also don't think he really seems to make that much use of his height - his balls tend to skid on, whereas if you compare with other bowlers at his height, they can get it to bounce of a good length and bowl a heavier ball. Unfair comparison I know, but compare him to Glenn Mcgrath for example who was of similar pace and height.
I doubt if he played for anyone else in the top 3-4 teams he would have got anywhere near as many chances in all 3 forms of the game (his test average is very poor as well for someone with over 50 matches) which makes me think there may be some kind of partiality towards him for some reason, or maybe India just have a dearth of half decent medium pace or fast medium bowlers. Umesh Yadav seemed promising, so not sure what has happened with him.