The scientific reason is that dyslexics lack pre-phonemic awareness unlike their non-dyslexic peers. They cannot manipulate the sounds in a word. For example, if you ask a dyslexic child how many sounds are in “cat” they will reply, "one". They cannot break out the three sounds C, A, T. They have to manufacture a spot in their brains for doing the manipulation of sounds before they can make use of the phonemic-based curriculums in most kindergartens and grammar schools.
The neuroplasticity of the brain is greatly more flexible the younger a person is as well. These games are aimed at helping young children develop the ability to break apart the words, manipulate the sounds and eventually rhyme words.