No idea. I suspect you're going to tell me that DB were the only provider available.
Correct
It doesn't however take the responsibility for the poor performance of the subcontractor away from TfW. I have every sympathy with staff in the company, and indeed I have personal friends who work for TfW. However to a passenger buying a service from a TOC, saying "it's not our fault, the locos that we hire in are broken" isn't an answer - their contract is with the TOC.
I'm not aware of the company having stated that anywhere. Publicly they are apologising for the trains not working and they have made it known that they are taking steps to rectify this. Privately I'm sure there are plenty of discussions going on between DB and TfW about how to improve things. There may well be contractual compensation in place but we'll never hear about that.
If there was only a single source of suitable motive power available you could well argue that the decision to continue to use LHCS rather than order additional new build trains was a poor one, however this is a different topic.
The use of a small amount of loco hauled stock was a franchise requirement for the original Keolis Amey TfW contract. At the time, DB 67s were pretty much the only suitable loco available, and also gave the advantage of low training requirements (the existing loco drivers only needed to do a 3 course for conversion from MKIII to MKIV DVT).
When the decision was taken to expand the amount of loco sets, the reasons above still applied.
It's also worth pointing out that by and large, the 67s performed pretty well on the MKIIIs (most of the issues of poor reliability on those sets was down to the stock rather than the loco in my experience). Now that the locos are being used much more intensively, and more of them are in use (meaning some locos have had to be brought out of storage), they're not performing anywhere near as well as they did before.
Hindsight is a wonderful thing, but it's easy to see how we got where we are.
The question now is, what comes next? When the MKIVs were first introduced their reliability was also appalling but they're doing a lot better now, after a degree of time and effort was put in to fixing them. Hopefully the same can be done for the 67s - or an alternative can be found.