IT does not have as much trade value as you think he does. Crowder is probably more valuable to a contending team because of his contract. This trade is really all about the Nets pick. Clearly Boston was not giving IT the max so he is expendable.
TP - Like Rondo, IT's value was evidently not that great. Not only does this trade show us the Danny never intended on signing IT to a long term near-max contract, but that Cleveland doesn't intend to either. They will likely make a big run with the IT and Crowder additions, even likely beating the Cs, but they won't be able to afford IT next year if Lebron sticks around...and why would Lebron stay with an apparent lesser roster.
This trade for Cleveland was in two parts: contend this year, rebuild with a top tier next year. For Boston (if we plan on re-signing Kyrie), it is for now and the future.