Could you be more precise? Which specific rifle are you thinking of?

Generally though, there's always been a problem with mounting scopes on military rifles for two reasons:

-Interference with the bolt handle.

-Interference with the stripper clip guide.

In WW1 in particular, there's also the issue of primitive optics. The Springfield 03 musket sight was an off the shelf product with (I assume) an off the shelf mounting rail. The sight is massive and appears in images that it was mounted on the side of the rifle to prevent interference with the bolt handle, As it does interfere with the stripper clip guide. Stripper clips were typically only used in military rifles, not commercial rifles so its likely that care was not taken to accommodate clips in the design of the commercial musket sight.

Another example of interference of a scope on both the stripper clip guide and the bolt handle is the 91/30 sniper variant: to mount the scope, bending the bolt handle is necessary.

Another example is the SMLE. The British MOD always insisted that stripper clips be used on the SMLE rifle, even in sniper rifles, so the scopes were therefore side mounted, on the left side to also avoid interference with the bolt handle.

I'm not a historian but a target shooting enthusiast so I hope my fairly general answer will help.