two shocks on the back?

like MAC said, just a different design , made to do the same job
I think in the beginning, they felt it would provide a better rode, due to there sport atv's used it
and I gather its cheaper for them to make, the bigger reason maybe?

I also preferred the old two shocks, to help level loads on one side of the rear rack , and I think they also helped better with down force on each tire better on bumps

but single shock rears have been out a long time, and most are happy!