Older mountain bikes are generally cheap and plentiful on the used market everywhere so they tend to be popular.
If you are looking for a city commuter I would consider steel over aluminum. It can be more comfortable and soak up pot holes and road buzz better then aluminum. If you get an aluminum bike I would recommend one with a carbon fork or steel fork. What is your concern with weight? For a city bike the only weight concern should be if you have to carry it up and down stair every day.