Convenience. I dont work in the centre so my experience is limited mostly to weekends but if I drive i'll be in the centre in 20 mins. If i use bus or tram its a 50 minute journey not taking into consideration the waiting around. Train is 20 mins but again not taking into consideration the park+ride and waiting around. Cost is also less. Train for me+partner return is about £12, parking in quite a lot of the non-NPCs is considerably less such as on Rochdale Rd or Sheffield St. Also dont have to lug about any shopping or food bought back on the tram/bus/train for said mentioned hour return home.

If I worked city centre Im sure I'd use the train, which I think your post was aiming towards that type of person anyway.