They are looking for people who will fit in to an integrated course and progressing in line with a very rigidly structured syllabus.
From what I've heard, integrated students are taught to fly as if they were flying an airliner from day one. An experienced PPL will have some general aviation specific skills and knowledge that may conflict with the course content and actually slow progress. On the other hand, someone who has a limited amount of experience and is familiar with the effects of controls may progress quicker and by getting those hours they have demonstrated a measure of commitment to an aviation career.
I'm not saying it's right or wrong, it just seems to be the way it is. I applied to the same scheme in 2003 before they added the 60hr limit. I had a PPL and 265hrs and got all the way through to the final interview. I didn't get sponsored but I was first reserve in case anyone dropped out. I guess I'll never know if it was my experience that made the difference or not. However, I have since gained my fATPL via the modular route and that extra experience has now lifted me above many 250hrs pilots in the job hunting stakes. What I have noticed is that recruiters tend to focus on pilots in certain bands of experience. The trick is not to stay in the gaps between these bands for too long!
Good luck,
Al