Humm I don't know..

People take drop bar bikes and convert them to flat bars and people take flat bar bikes and convert them to drop bar.. It's all good in my opinion as each person can and should pimp and trick their bike out how they want it..

Personally I would not do either but I also don't have an issue with anyone who does..

To the OP's question it could be a ton of different things as to why one bike was smoother than the other. Gearing, tires, crud buildup on the drivetrain, bad hubs, brakes dragging ect.. It's really kind of hard to give an answer..

If you still have the Nashbar bike then maybe have a mechanic look at it..