Some want to be small and lightweight, e.g. Dillo, so I can imagine them rejecting such features altogether.
Others don't have features like DOM, Javascript, etc. so there's not much they could offer that "view source" doesn't already do.
Others don't have a suitable interaction model, e.g. w3m is effectively a pager for turning HTML into ANSI escape codes; conkeror is keyboard driven so would need a radically different UI for it to be effective; etc.
Others, like Netsurf, Konqueror and the Webkit wrappers (Midori, Arora, etc.) would probably like a dev tools feature, but don't have enough developer power to implement one.
Others don't have features like DOM, Javascript, etc. so there's not much they could offer that "view source" doesn't already do.
Others don't have a suitable interaction model, e.g. w3m is effectively a pager for turning HTML into ANSI escape codes; conkeror is keyboard driven so would need a radically different UI for it to be effective; etc.
Others, like Netsurf, Konqueror and the Webkit wrappers (Midori, Arora, etc.) would probably like a dev tools feature, but don't have enough developer power to implement one.