There are other factors but I agree money was probably the driving force. But joining Spurs opens other doors to career progression.
Even at Under 21 level being at a London or a big 6 club increases the chance of International recognition.
You ask why top championship clubs havent signed him, that is sample. They want proven talent as they want promotion not somebody else's project.
Time will tell if he has chosen the correct path but I agree with RF99 in that if he had stayed here he wouldn't have played the number of games in the championship that he already has. JDT played him only because he had to, if we had signed Batth the season before, like we supposed to, then he wouldn't have played that number.