Because you cannot see a "need" for large capacity magazines in no way should limit a persons right to have them. That is your short coming.
In reality, the reason for the desire to have large capacity magazines is not the same for all gun owners. I want a 13 round magazine for my Browning H.P. because that is what has traditionally been supplied with it since WWII. I want a large capacity magazine for my AR-15 because if I have to use it in the capacity of militia vs. treat to the security of a free state, I anticipate multiple targets (that is why current U.S. military weapons are supplied with "large magazines". I want a "large capacity", 20 round magazine for my M1A because when I use it for NRA High Power matches, it is useful for supporting the rifle with the off hand.
Aside from all that, the former restriction on large capacity magazines did not provide any empirical evidence that the restriction of such had any positive effect on the crime/murder rate.
A last point, there is not real disadvantage to having a less-than 10 round magazine in some instances. The Colt .45 ACP and copies traditionally have been supplied with 7 round magazines, and with practice, a knowledgeable gun owner will drop the spent magazine and reload a full one in a 2 seconds or less.
In all, the restriction on "large capacity" magazines is another ingenious solution to a non-existent problem.