Long story short, even if the railway was reinstated from Sandy to Trumpington, the P&R and guided busway would have to be obliterated.
I believe the intention is also for EWR to serve the "Cambridge South" station, if it goes via Trumpington it wouldn't connect to the station that is currently proposed.
Coming in from the north close to the A428 allows the railway to serve Cambourne and the large housing estates that will be built up there, as well as allowing an extension to Stansted from Cambridge South as soon as there is more capacity - an Oxford/MK to Stansted service would likely be popular as it would avoid London and the Underground, even if it is not all that much quicker