I'm betting that Elfangor had been plotting to slip humans the ability to morph at the earliest opportunity, in order to hinder the Yeerks.
(I understand that he had secretly spent years on Earth, so knew that humans could handle morphing).
The Invasion: Elfangor and the 5 kids must all touch the cube in order to give the 5 kids the morphing ability.
The Discovery: The Animorphs AND David must touch the cube to give David morphing powers.
However, I have read a book in which a buffalo and an ant got the ability to morph merely by touching the morphing cube, without any action by the Animorphs! The buffalo proceeded to morph Assistant Principal Chapman and Visser Three!