Because soap can be a cruel cruel mistress
Unlike most products, which are fairly inert chemically, soap is highly alkaline and humans developed it to get things clean - like stains and stink out of clothes and skin. So some colors and scents just dont work like you think they should.
You really do not need to post basically the same question in two threads. You already brought this up in the Coloring Thread. Most fruit powders do not play well with the ph of soap as far as coloring goes