It's a really, really simple one: many liberals do support gun rights, and many others are on the fence about the issue. When we attack "liberals" as some sort of monolithic group, we will very likely alienate these people. Keep in mind that we aren't just preaching to the choir here -- we are representing gun owners to the public at large.
I agree. If someone is anti-gun, does it really matter what political party they are affiliated with regarding your 2nd Amendment rights?