Why Pilate? Because of the his long rule

[Giuseppe]

Because, after Valerius Gratus, he was the only Roman governor who ruled more time than any other Roman governor of Judea:

Coponius : 3 years.
Marcus Ambibulus: 3 years.
Annius Rufus: 3 years.
Valerius Gratus: 11 years.
Pontius Pilate: 10 years.
Marcellus: 1 year.
Marullus: 4 years.
Cuspius Fadus: 2 years.
Tiberius Julius Alexander: 2 years.
Ventidius Cumanus: 4 years.
Antonius Felix: 7 years.
Porcius Festus: 2 years.
Albinus: 2 years.
Gessius Florus: 2 years.

Why Pilate and not Valerius Gratus? Because Valerius Gratus was for 2 years governor also of Syria. Probably this made the difference between him and Pilate, in the choice of the first euhemerizer.