In comments, John Lawler wrote:
Because where, when, and why have very limited use as relative pronouns. They are most common in headless relative clauses (or disjunctive embedded question complement clauses, depending), like the last two examples you give, which are pseudo-cleft constructions derived from We go on vacation in January and You are grounded because of the broken window in order to emphasize the last NPs, not to identify them the way an a restrictive relative clause does. It's not unusual for oblique wh-words to have limited usage; how may not be used as a relative pronoun at all.
By the way, the proper way to "research relative pronouns" is not to search websites or the British Council or other Authorities, but rather to assemble a corpus of English sentences and see how the relative pronouns are used or not used in them.
When can only be used to start a relative clause modifying a time word, and where a place word; why can be used only if reason is the antecedent. That's why these pronouns are used so comparatively often to mark headless clauses; there are so few antecedents they can have that they can be safely implied and therefore not expressed.