Because if they send "humanitarian help" there it'll just end up in the hands of the separatists. What a concept, right? Why would they not want to send aid to their enemies? Seriously, your shit keeps getting more and more retarded.
I guess if the separatists managed to intercept a red cross caravan full of pasta and toilet paper, this might put the balance of the war askew, since the rebels are totally in a besieged state, having their back to Russia borders, so if they got their hands on humanitarian help, it will greatly improve their battle efficiency and morale.
The fortunate population caught in the crossfire which are naturally fed by both Ukraine and rebels in the middle of the war zones would frown upon such assistance, and possibly even feel insulted.
Also, it would be the first time humanitarian help is ever being the subject of corruption and trafficking, in this case the only logical solution would be to stop any such moves.