Agreed it is much more expensive, but links a far bigger population.And... that makes it much more expensive. For example, to develop an alignment serving Luton Airport Parkway and Stevenage means building 11km of line that would be almost entirely in tunnel, deep cutting or Viaduct. Plus three grade separated junctions. That would be £1.5bn plus. And it doesn’t solve the issue of capacity from Bedford to Luton, or Stevenage to Cambridge.
Don't think it is much of an issue capacity wise. On both lines it would link to the slow/relief lines and in the case of the ECML by grade separated interface at both stevenage (courtesy Hertford Loop) and Hitchin, on a stretch that has inevitable spare capacity due to the Welwyn viaduct and Tunnel.
On the MML the north end of Ampthill Tunnel to Luton Airport Parkway has capacity, it is Harpenden Junction southwards where it gets messy.
Its too late now, the option has been rejected, but it is to my mind a missed opportunity, and I don't see hordes of people wanting fast trains from Cambridge to Bedford and Bletchley.
The Stanstead via Hertford service that AOwen derided was part of that rejected option