The WT Society has convinced JW's that they are the one true religion and everyone who is not a JW will be destroyed at Armageddon. In the mind of a JW any poor or homeless person who does not become a JW will be killed at Armageddon anyway- So, why waste their money on a soup kitchen when everyone in it is just going to be killed at Armageddon anyway?
Now, if the WT Society should ever sponsor a charity for the poor they would be saying in essence to JW's, that the end is not around the corner and maybe JW's are not the only ones who God might look favorably upon. JW's might begin to see charitable work as having more value than door-to- door work , too. It is the start of a slippery slope that the WT Society does not want to venture onto at any cost. I am sure there are more reasons, too.