The 175s did have gauge clearance issues when first introduced and clearance was difficult to obtain so they were restricted to the following lines: Holyhead to Chester, Llandudno Junction to Llandundo, Chester-Warrington-Manchester, Chester-Altrincham-Manchester, Chester-Crewe-Birmingham, Manchester Airport-Oxenholme, Preston-Blackpool North, Oxenholme-Windermere, Carnforth-Barrow and Manchester-Wigan-Preston. Given that there wasn't a huge fleet of 175s and there were clearance issues not getting clearance for every line made sense.
Someone said something about the steps by the doors being adjusted before they could go down to South Wales. I don't know if there is any truth in that.