There are several reasons:
1. Potential future upgrades - A single WAP5 can definitely haul a 18 coacher at 150 kmph, but longer rakes ( like the existing BCT Raj for example) cannot reliably maintain such a schedule with a single loco. The twin WAP5's will be necessary if the rake length has to be increased.
2. Better acceleration and braking - Trains take a lot of time to accelerate at higher speeds. While a WAP7 or a WAP5 can accelerate very fast at lower speeds (<110kmph), beyond that they will take considerable...
more...

time to accelerate from 110 to 130 and above ( In this case, WAP7 will struggle even more at higher speeds). So, even slowing down from 130 to 110 and accelerating back will cause delays in this schedule. The twins are mainly being used to ensure that lesser time is wasted in the high speed sections. (At lower speeds, twins or single doesn't make much difference). Also, twin locos provide more braking power. So the train does not waste time in deceleration and can run at MPS even if it's very close to the next halt. Essentially this helps the train behave more like an EMU.
3. Reliability - BCT Raj was downgraded to a P7 link after the P5 link was observed having problems in wet conditions on graded sections. While this problem has largely been tackled, they are still hesitant to upgrade it back to the P5 link. Twin WAP5's will negate these issues effectively, and they will also be more reliable in that, even if one of the loco fails or struggles, it can just be turned off and the train can continue it's run with minimal delay.
A WDP4 can handle such a run, but it would need utmost priority. IT won;t be able to cover up delays if there are too many PSRs/TSRs. Diesel locos inherently take a longer time to accelerate, and more so at higher speeds. So, if there are too many speed restrictions, a DP4 will struggle to maintain this schedule.