Not because he isn't a very good player, he is, but because it will take some resources to get him and there are other options that may make more sense. For example, I would like to see Pietrus re-signed as a better defensive option who can still hit the 3. I also want to see how Kris Joseph pans out because he has looked impressive in SL play. Finally, if the MLE can be freed up by S&T to get JET, I would rather use it to get a big man (somewhat unrelated thought there).
In any case, I just have feeling Ainge is not going to great lengths to get Lee. I suspect he likes a lot of the young guys and he may have an eye towards the future.
If all the pieces fall into place, this team can win #18 without Lee and still have a solid young core for the future:
Rondo, Green, Bradley, Sullinger, Joseph (maybe Melo too, not sure yet)