We found out in the original series of films that Ash was The Chosen One in terms of stopping the Deadites. And now in the upcoming Ash vs Evil Dead Season 3 we're going to find out why.
Speaking to Dread Central Bruce Campbell had this to say on the matter
"The mythology’s going to kick in now, which will give us all an idea of why Ash was chosen– this average idiot… this fool was written of in an ancient book,” Campbell teased. “So Season 3 is where it all will come to a head – the ultimate mettle of the average man will be tested by these demonic forces, and if man fails, then evil will rule the earth – those are the stakes.”