In a word MONEY. Angel Trains are asking too much for them and FGW▸ or anyone else WONT pay what they are asking. Personally I feel FGW should work something out with Angel trains and as an act of goodwill and get them running on Cardiff-Portsmouth so the displaced 158s can be used elsewhere on the network .....
I heard a quote of 15 million pounds per year for FGW to retain the Adelantes - I'm not sure if that was for the whole fleet, or enough for the Cardiff - Portsmouth, nor whether it was the total cost or the difference over and above using 2 car 158s for the service. If anyone can help quantify this, it would help in the understanding.
The comment was also made that the TCO (total cost of ownsership) of an Adelante is the same as the total cost of owenership of a 125, even though they only have a half of the capacity.
It's not in anyone's interest to have expensive resources (trains) sitting in sidings doing nothing when they could be earning money for both their owners (Angel Trains) and an operator (such as FGW). But while the two parties wrangle and negotiate and play brinkmanship, it leaves the people for whom passenger trains exist in the first place - passengers - fuming on the platform. I am inclined towards blaming the system more than any of the individual companies. When the Adelantes come back into use somewhere, we should all take a look at the solution and ask "why couldn't they have got their heads together and come to that conclusion before taking them out of service?"