can some1 explain me why statikk shyv is prioritize by SKT on ashe?

You trade the late game strength of PD so you can reach your IE + Shiv Powerspike earlier.

Holding onto Avarice Blade is also a good comeback mechanic if you get behind.

I'd never get Shiv on the basis of 'needing waveclear' though because most of the time ADCs just play Tennis with creep waves after laning phase anyway. If you really need to, kill the Small Minions fast, then PURELY last-hit the Melee minions then regroup mid. Slow pushing a wave will either draw the Enemy ADC deep and therefore make them vulnerable, or the wave will build and create pressure if you siege mid while it simultaneously threatens a side lane turret.

You get Shiv when you're behind or you need waveclear.

I think Shiv proc's if your regular auto crits right? If it does, all Ashe has to do is use her ult to initiate and her first auto will have a nice burst.