For expats, it's usually the 3 D's - divorce, debt, drink - they relocate to Asia (in this instance Jakarta) to start again, etc etc, better quality of life for their $, cheap beer, yes sadly also Raja Minyak syndrome looking to feel handsome again with 20 year old girl etc
You can hardly say Jakarta is rife with lucrative jop opportunities or even welcoming for entrepreneurs - in fact most of them are leaving and fed up with beaurocracy and corruption here.
If you mean why Jakarta and not Bandung etc, well yes it is the capital and no other major city in Indonesia comes even close to the finances within Jakarta city.
Even the bars now are full of brondong at the weekends who fly in from SG because it is cheaper to party here than in HK SG etc and possibly easier to find a friend. Ironically from an alcohol pov it is not that much cheaper to party here than over there unless you are pretending to be a baller and open bottles, which is pointless in HK and SG anyway.
But each to their own, if you live and work in the same area and have no family, then traffic etc is not a factor. If you are smart with your money (e.g. rent) then you can still save a lot.
But as you can read on the FB forums when expats are talking about how $2,000 a month is good money here (WTF???) then the standards have fallen.
$2,000 a month is less than the UK average wage so to pack it all up and move to Asia for less than you earnt in UK is a warning sign that they are not here for career progression and it is hardly all palm beaches and clear blue sky here.