Please don't say "hard work" or "ability". Ability determined by whom? One of my old employers used to promote people more "qualified" because they had a special training or certification. After enough digging I discovered people could only get that special training and certification if their employer sponsored them to get it. In short; the "qualified" were made by the higher ups but the rest of us were told they got it from merit. Obviously unfair.
So why not promote based on seniority?