In my view the following mistakes made by India are the reasons.
1) Cause: Poor Scheduling of tour.
Effect: Went for tour with just one practice match.
2) Cause: Lack of Killer Instinct:
Effect:Failure to seize opportunities at crucial moments.
3)Cause: Lack of Quality left arm seamer(s): An area where English team (or any team) has shown some weakness in recent history. (Remember R.P.Sing's success in last series under captaincy of Dravid won by 1-0).
Effect: After injury to zaheer teams bowling department was in disarray.
4) Cause:Non Availability of 3rd specialist opener - Blunder by selectors.
Effect: Dravid was made opener and team combination was hit badly.
5) Cause: Injuries at wrong time: (to Sehwag,Zaheer, Gambheer, and Yuvraj, Harbhajan)
Effects: Absence of quality players with experience.
6) Cause: Recent Successes:
Effect: Reduced hunger for success, (the English team was taken lightly under swinging conditions)