So Meru is less high and has fewer visitors yearly because it’s not the highest mountain. Meru doesn’t have Glaciers. Something that the Kilimanjaro does have. On the other hand, does Maru have more rough landscapes and the chances of seeing wildlife are bigger!
Meru can be conquered in three days (the Kilimanjaro in a minimum of five days) and the climb is less difficult most of the time.
Climbing Meru can be perfect acclimatization before conquering the Kilimanjaro.