Drop spindles don't spin continuously, which gives the wheel an uptime advantage.
Even modern spinners who prefer a drop spindle often have a wheel on hand for plying. Spinning a thread out of a blob of fluff can be engaging and interesting, while plying is a tedious process that's hard to get wrong without falling asleep in the middle. There's not really an art or skill to plying. A wheel can knock out this boring part of the spinning process much more quickly.
Even modern spinners who prefer a drop spindle often have a wheel on hand for plying. Spinning a thread out of a blob of fluff can be engaging and interesting, while plying is a tedious process that's hard to get wrong without falling asleep in the middle. There's not really an art or skill to plying. A wheel can knock out this boring part of the spinning process much more quickly.