You didn't have to fly PRG-TXL-CDG-ORD; you could have flown PRG-LHR-ORD [BA] or PRG-HEL-ORD [AY] or PRG-MAD-ORD [IB, though this is only operated seasonally] or even maybe PRG-DUB-ORD [EI].

I guess that this particular routing just happened to be cheaper than other, more direct routings on this particular date and at the particular time you bought it - or if you knowingly introduced the constraint that you had to pass through CDG [going via CDG is not an obvious routing for a oneworld PRG-ORD flight]