Well it's because you're not killing enough monsters. Your items, runes, talents, symbols etc. are not as good as others, therefore the average amount of monsters you kill is lower than others which directly affects the amount of shadows you get in an hour.
You simply can't say something isn't fair because you didn't work hard enough on your character. It's like me coming here to say "Hey, why am I not level 19? I spend a lot of time so I should be 19 immediately!" I'm sure you see the connection here.