How come Sona and Udyr can get decent skins after their ultimates, but Ezreal can't?

Sweetheart Sona and Definitely Not Udyr are both good, quality skins released after the two of them got ultimate skins.

Meanwhile, Ezreal has gotten THREE different 750 skins that do nothing but change his base model since Pulsefire got released. In fact, Pulsefire is his ONLY skin (out of eight) that has any different particles at all, and thus is the only one that costs more than 750 RP. It's not like Ez has any shortage of particles to play around with, so why?

(On an unrelated note, Pulsefire is basically a slightly glorified legendary skin at best by today's standards.)