Because the team costs something like 40mil a year in wages, City spend £180mil, simple as that. City get an injury, they put another 20mil player in the starting line up, Wigan do, they put a free transfer who is a bit crap.
More to the point, its been half a mil difference per place you finish in the league, to come much higher up the table would cost 20-30mil in wages, but there really isn't 20-30mil in extra income from doing so.
they don't have the squad size or quality to put in sustained great performances, as such, when a team knows they can't come much higher than 18th, you get a motivation problem, where they only try when its really important.
ultimately a team with a small squad and a severe drop off in quality with back up/rotation/youth players, can't compete well in the league, thats football, money talks.
http://www.financialfairplay.co.uk/latest-news/wage-and-the-premier-league-scaled-table
there are absurd teams like Villa, QPR, to a degree Sunderland, Villa had a high spending regime and used to compete in the top 7, QPR are just a joke, a complete joke, Sunderland have overspent and constantly sign players. Wigan run on a phenomenally small budget, and they consistently spend less than the vast majority of the league and teams spending twice as much on wages have been relegated multiple times. You need to spend more to be higher in the table, but spending more doesn't make it certain, you can spend badly, very easily. Blackburn, Bolton, QPR wage bills are a joke, Newcastle and West Ham went down spending similar amounts, 60-80mil a year for those guys at various points.
Wigan spend an amount that most managers and most squads with that kind of spending WOULD get relegated. People just don't seem to get, Martinez has one of the best value teams achieving more than most managers would with similar spending.
Wigan's finally made a small £5mil profit before interest/tax and other stuff last year, but a loss of £7mil after that.
Biggest problem for Wigan is support/stadium/ticket/sponsorship income, such low attendances, means all those bring in very little and they are mostly about the TV money, if they can survive a year as such the tv money increasing by sucha large amount should help Wigan more than basically any other club.
If they go down, they can be proud to have stayed up for so long on such a tiny budget.