Its a lot quicker to run the bus from St Austell to Newquay than from Par and it gives better onward connections on to the mainline. I also suspect most local travellers go on to St Austell anyway as there is hardly much to detain them in Par! ( with apologies to Parisians).
I saw the initial comment and thought "sensible for passengers to Newquay - but doesn't that make it a long way round for passengers from Par to Luxulyan, for example?". I suspect my thoughts were a classic example of looking from my armchair in far, far away. Luxulyan passengers may be rare as hen's teeth at the moment when you look at normal numbers, and the added distance really isn't that much when you look at the geographic layout