The most important reason why globular clusters are useful for finding the speed of the Sun in its orbit around the galaxy is that globular clusters:

are large and bright, so they can be seen through the obscuring dust in the galactic disk.

on average do not revolve around the galaxy, so they provide a "stationary" frame of reference to which the Sun's motion may be compared.

on average rotate around the galaxy at the same speed as the Sun, so they provide a frame of reference that is "stationary" relative to the Sun.

are distributed uniformly around the galaxy, so they provide a well$-$defined grid of measured locations around the galaxy.