Quay Park is basically a giant property development play.
Sometimes they work out, especially if the property market is running hot when all your new shiny apartments are ready to sell.
Many times it all goes horribly wrong. Rising interest rates, a dropping property market, project delays, unforseen cost blowouts etc. The need to dig under the railway line which doesn't sound like it's been fully costed, screams potential cost blowout.
Talk to any insolvency specialist and likely over half of their book at any time, is failed property developers. Council would have all sorts of trained staff - planners, engineers, property experts etc - spending hours poring over the different proposals. Doesn't really sound like they were hugely impressed with either, and they report that back to the elected representatives. But EP is analysed as the lower risk option, and that's the main reason why sadly the Councillors have gone with it.