Although Peter Quince seems to be the designated leader or director in this group of workmen, what draws our attention to Nick Bottom?

One simple explanation is that Bottom has the lion's share of the lines.  Also, of all of the craftsmen, his personality is the largest.  In the first scene with the rude mechanicals, even though Quince ostensibly runs the "rehearsal", Bottom dominates.  He wants to play every role and gives an example of his brilliant acting for each one.