The short answer is that a 10mm spacer doesn't always leave enough room for the vehicle’s hub to fully seat within the spacer.
The long answer is that even on two identical cars, the hub lip height is typically exactly 10mm, but can vary by as much as 1.5mm. If the vehicle’s hub lip is taller than 10mm, that is to say taller than the spacer itself, this leaves no room for the spacer’s hubcentric lip to be attached to the mounting surface. This can prevent the spacer from seating flat against the hub, and will cause the spacer to “float,” even by half a millimeter. As you can imagine, when torquing down the wheel this causes the spacer to bow, and will ultimately cause wheel stud/bolt failure which can result in severe vehicle damage or injury.
Other manufacturers do produce 10mm spacers, and they often clear without issues. Most drivers seeking a 10mm spacer feel that the extra 2mm is well-worth the peace of mind in eliminating the possibility of an improper seat, and we’ve found that there is rarely a case where a 10mm spacer allows a wheel to clear perfectly, and a 12mm doesn’t.