It's the retailers that tell publishers what format they want product.
In Europe shelf space is at a premium and game boxes are small DVD cases, retailers see value in small and light and know that almost everyone of their customers have DVD-ROM drives and recon that cost saving versus addressing entire market are fine. Distributors love this because one DVD is cheaper to make than three CDs, and they only have to put one disk in box (packing is therefore cheaper too).
In USA retailers believe that customers think large boxes with lots of disks equals better value for money product "it's got to be better it's on six CDs". They also don't want to miss out on any possible customer who may not have DVD-ROM drive.
That's the difference, simple when you know isn't it?
As mentioned in HL2 on DVD in UK thread, Konami are DVD-ROM only too in Europe; and the number of titles in this format grows by the day.