In the Ellimist Chronicles, it's stated pretty clearly that Crayak selected the Earth solar system as their last chessboard / battlefield, partly because it was just where they ended up next in the universal geography, and partly because Earth had a superficial resemblance to another (fictitious) planet that Ellimist had defended in the games back on Ket. Everything that happens in the series, from Elfangor's crash-landing, to the Time Matrix business, to the current Animorphs, was orchestrated by either Ellimist, Crayak, or to some extent both, with each having an ulterior motive for some move they agreed on making.
Ellimist himself stated that he valued Earth and found it beautiful because of its sheer biodiversity and geographic diversity. The Yeerk, Hork-Bajir, and Andalite homeworlds all have much lower biodiversity and, for the most part, all three are limited to a single habitable ecosystem. Considering both the Hork-Bajir and Andalite worlds were engineered respectively by the Arn and Ellimist himself to be what they are at the time of the story, and considering the Ellimist never lays claim to having started any new life forms on Earth, beyond encouraging human development and being the 'great-great-grandfather' of domestic dogs, through the Pemalites and Chee, it could be argued that Ellimist simply finds it interesting just how varied life became on a planet where sentient life and biodiversity did not have to be engineered into existence.