Primarily, single cams are said to require no cam synchronization as do two cam systems. The other advantage has nothing to do with archery and everything to do with manufacturing costs.
Margins are tight in the archery business, particularly for bows.
The cams are one of the most expensive components to make since they are fully machined. If you only have to make one cam and replace the other with an idler wheel, your costs have just come down dramatically. Beyond this point we start getting into the controversial. Mathews has gone to the very fringe of ethics to promote their bows. I won't bore you with the engineering details, but their bow design has an inherently higher degree of hysterisis than two cam bows.