Advance bookings which were significantly poorer than expected, from what I've heard.
If that is the reason perhaps it's because Wizzair is not a well-known brand in the West Country and, apart from the BRS website and an initial publicity blurb in the local press when the route was announced and even that was very brief, I've seen no advertising for it.
They didn't give it long (about seven weeks) and the route wasn't due to start until mid September.
Ryanair's routes to Poland saw monthly load factors in the low to mid 90s% last summer and into the 80s% in winter, as did easyJet to Krakow.
When easyJet operated the Warsaw route in 2007/2008 they averaged nearly 90% load factors but axed it reportedly because of the high cost of operating into WAW. They stopped their other WAW routes around the same time.
Unless the continuing recession and volcano problems have suddenly impacted adversely on Bristol's Polish routes across the board it seems odd that Wizz cannot at least find a market in terms of bodies on seats.