The main reason for this delay is money. Network Rail like Heathrow work on fixed investment periods. The money / budget / cash-flow for these seems to be relatively fixed. The GWML project has gone over it's budget and is using money from other planned projects in that period. It means no money for the T5 link so the T5 project gets moved back into the next investment period.
Don't expect logic such as it would cost more to do separately to figure widely in that decision.