Because they wouldn't fit. Voyagers are too narrow to do it.
3+2 was the plan, by the way, not 3+3.
3 + 2 ? Surely not, may as well have run the service with a 4 car 150!
I’ve used voyagers many times over their lifetime and I’ve come to appreciate them much more when comparing them with newer trains.
When not crammed full they are very pleasant to travel on. For example this weekend the air conditioning was in fine working order, The ride was fast and smooth, the catering staff very friendly and helpful, the seat was comfortable, the lighting was warm and subdued, all very calm and relaxing.
There are things which could be better, many of the individual Reading lights don’t seem to work, the vestibule door will grab you tightly if you don’t manage to rush through it with the kids and luggage within 3 seconds, overhead storage could have been bigger.
Overall, lovely journey as I got lucky with not being on an overcrowded train.