Out of interest, what is the reason for this difference?
Strength, as mentioned above.
The front could be 10mm (I dunno why it isn't) but the back cannot be 9mm.
The highest forces in a bike frame are at the right chainstay/chain and rear axle.
Under tension, the rear axle bends in a bow as the chain pulls forward on the sprocket.
The narrower the distance between the two drop-outs (called the OLD for over locknut dimension), the smaller the bow and conversely, the opposite.
Another factor that affects the bow is the location of the right side wheel bearing. If it is close to the locknut, it supports the axle better than when it is moved further inboard. This became very obvious with the advent of 8-speed freewheels (not freehubs). This was just one step up from 7-speed but the OLD changed from 126mm to 130mm and the right bearing was now just too far inboard for what a threaded, 10mm hollow steel axle could withstand and we had plenty of "bent" axles as a result. I put bent in quotations because these axles weren't bent, they were cracked. The cracks weren't easy to see but could be shown by removing the axle, blocking one end of the axle/tube, immersing it in water and blow on the other end. Bubbles would emerge just there were the outboard bearing sat on the axle.
The remedy for this was the freehub, which had a different design and bearings further outboard. It solved the problem even though it retained the wide OLD of 130mm (or more on MTBs) and 8-speed cassette spacing.