The main reason is location. Newcastle, although its the largest non-capital city in Australia, is not as popular a location as Western Sydney. It works the same with the top universities, for example ANU (in Canberra) vs USyd, UTS, UNSW (all sydney unis). The UAI cutoff is all about supply and demand... more people allready live in sydney and want to go to sydney than newcastle, and so the UAIs in syd universities are generally higher... You can see this within the same business courses at UWS that are held a 3 different campuses... parramatta campus always has a higher cutoff for the same courses as the othetr UWS campuses because it is in a more desireable location...
As for a comparison of the business courses... I don't know which would be considered better, I havn't heard anything bad about newcastle business, if thats any consolation?