I think some of your comparisons are a bit unfair there and you could add plenty of nuance to some of those cases if you wanted to.
I would actually say the biggest factor in all those strikers you mentioned (apart from maybe Vardy) is that the manager they are playing under and the tactics their teams employ have a huge influence on their output. All those you mentioned have had their best output playing under top managers. DCL under Carlo, Solanke under Iraola, Bamford under Bielsa. Up until Emery came Watkins' PL output was not a great deal better than Delap's this season.
They've then fallen off when playing under bum managers (Ange, Dyche, Benitez, Marsch etc)
Delap is younger than all of those when they had their good patches too