My perception, for what it's worth, is that a number of people, starting with John Logan, the writer/creator of PD, had other projects to move onto. Logan had mentioned in interviews during the first year that he had planned a 3 year arc. In a couple of interviews that I have read since the finale was broadcast, he said that during the course of working on season 3, it had become clear to him that the end of the arc was also going to be the end of the series. Apparently the principal players involved, including Eva, agreed.
Pretty much the same thing with the "cancellation" of DS although it's never been directly stated that Dan Curtis was the one who ultimately made the decision to pull the plug so he could direct NoDS and then move onto other things. That is the only explanation that fits the chronology and the other pieces of info that have become available over the years. He certainly had a full plate as soon as the series ended and he pretty much kept gone right on for decades afterwards.
G.
