Used to express that it's understood why 'A' would be the case but not 'B'. It's like saying 'Well, I can understand it for A, but for B...?'
Long explanation:
The はいざ知らず grammar point is used when making a comparison between two things: the first being a logical or natural consequence or situation, whilst the second is more surprising or unexpected. It is often translated to English as 'Well, it's understandable for..., but for...' or 'I can see why...but ...?'