Because twice as much* better rolling stock would cost a lot more than too few ageing Voyagers.
* That being roughly what would be needed to totally eliminate overcrowding and give room for growth from suppressed demand, I reckon.
There's currently 312 coaches at XC, if we double that to 600 coaches then XC could have 60 units with 10 coaches of 26m (similar to class 80x) with over 600 seats per unit.
Given that there's 44 diagrams, XC could add a further 7 diagrams (which would likely mean longer routes).
Even on the 4+4 services (400 seats) that's at least a 50% uplift in capacity, for the services which are currently 4 coaches (200 seats) that's at least a 200% uplift in capacity.
As has been highlighted before, not all 5 coach trains are created equally.
In fact a 304 coach fleet with a mix of 5 and 9 coach sets would give an in service seat capacity uplift of 30% over the current fleet.
It's also worth noting that even just adding the 6 sets currently in storage (assuming 85% usable fleet) could result in:
12 diagrams of 4+4
8 diagrams of 4+5
24 diagrams of 5
Note that this is at any given time and so some of the 5 coach diagrams could be run as 5+4 in the core as some ofthe 4+5 diagrams become 5 coach diagrams beyond the core.
Either way, having the potential to eradicate the use of 4 coach units as single sets and 45% of diagrams being pairs of units is likely to help with capacity issues (as long as all the pairs of units don't have a rear unit out of service).
As to costs, with lower track access charges, far fewer pairs of units, lower fuel costs and slightly fewer coaches a mixed 5/9 coach fleet of 80x or similar units wouldn't necessarily need much of an uplift in passenger numbers to justify the new build cost.