I doubt bidders are proposing electrification, however it could be that if new units to replace the DMU's are being not proposed that there is some push back by the DfT to see what the extra costs would be for the franchise to do so and the timeframe in which that could be done. Just as, with the delays in electrification elsewhere, a load of 158's/159's being released before 2020 could be very useful in getting several other franchises out of a hole.
Given that there are a significant number of other franchises that have 158's it would mean that they could be split between lots of places rather than having to use them to cascade other units. They also are likely to have a relatively short lifespan post 2020, so could be ideal in filling in for overrunning electrification.
Of course it could just be very hard to separate the two in terms of being able to show a clear winner and so want more time to ensure that they have it right.