As the article explains, it grew to this size under very favorable circumstances: forest fires keep burning down competing conifers, and a shift in climate some 10,000 years ago made it so separate aspens couldn't really flourish.

Also in the article it mentions other clonal groups in Utah and elsewhere.

Interconnected stands of Aspen are actually common in areas where Aspen trees grow. Pando is just a particularly old and huge one. In the autumn, you can sometimes tell where two separate stands share a mountainside, because the clones tend to turn yellow all at once, and you can see that one side of the mountain is still somewhat green while the other has turned yellow. I lived in an Aspen forest in Park City that was like that.