Cricket is just like any other profession. You can't force the players to turn down a high paying offer (just like the case with any other career). The salary levels in IPL is 10x or 20x of what the players receive in international matches. Given this, can you really force the players to opt for international matches, when the IPL is ongoing? It is extremely unfair. Some of the cricket boards, such as the one in South Africa has done this and in the end many of the players were forced to take retirement.
Then all boards less PCB have to make a consent that there will be zero bilateral series during the whole IPL duration. On the other hand IPL can be completed in one month easily, as we have one match going on per day. Two months is quite a long duration and its difficult for some boards to stop there cricket for two months.