Once they were selling a large enough quantity to begin with they can gamble their profits on overstock for future production, wiping out future supplies for competitors... at which point it's more of a guarantee, if there are multiple viable products and you remove all but one, consumers have no choice.

These are separate things (getting to the point of large scale production and blocking your competitors), e.g they could have played fair with the same initial success without excessive overstocking, allowing their competitors to continue competing and letting market forces decide, and letting suppliers gradually match current demand.

In short, there is no advantage to overstocking to such an excess other than to block your competitors. But you can only do so once you have a foot in the door and enough money to do so.