There are many stepper motor manufacturers in the market place, but there are even more stepper motor driver manufacturers. The crowded market place provides many standard product options for customers to build their motion control system just the way they like it.
However, if the best possible performance is desired, it would be difficult to beat a tested combination of a specific motor and driver from the same manufacturer. By combining R&D knowledge from both motors and drivers, unique product features can be developed to improve performance further.