Both give a reason, but what comes next is different. Because is followed by a full clause (subject + verb). Because of is followed by a noun or noun phrase, with no verb.
Because vs. because of
because — Maya was late because the bus was delayed.
because of — Sofia was late because of heavy traffic near Ghafwick Station.
Not ""The event was canceled because the storm." (no verb after because)" — say ""The event was canceled because of the storm."". A bare noun needs because of; a full clause with a verb needs because.
Try it: Mei: Why did Yasmin miss the call? Lani: She missed it ___ a problem with her phone. Which reply is correct before a noun phrase? (because of / because / because that) Answer: because of — 'A problem with her phone' is a noun phrase with no verb, so it needs 'because of'. 'Because' would need a full clause (because her phone had a problem), and 'because that' is not a real phrase in English.
Since (known reason) and due to (formal)
Since can mean "because" when the reason is already clear or shared. It often begins the sentence. Due to is a more formal version of "because of" and, like it, is followed by a noun phrase.
since — Since Amina has the keys, she can open the office.
due to — The outdoor class was moved inside due to bad weather.
Not ""The train was delayed due to it snowed." (clause after due to)" — say ""The train was delayed due to signal problems."". Due to takes a noun phrase, just like because of, never a full clause.
Try it: Wei: Why did the clinic close early? Farid: The notice says it closed ___ staff training. Which reply is correct and formal before a noun? (due to / since / because) Answer: due to — 'Staff training' is a noun phrase and the tone is formal, so 'due to' fits best. 'Since' and 'because' both need a full clause with a verb (since the staff had training), so they do not work before a bare noun here.
The reason why
The reason why introduces an explanation, usually before a full clause. It signals "here comes the cause" and is useful when you want to make the reason the focus of the sentence.
the reason why — The reason why Farid left early was that he had an appointment.
explain the reason why — Amina explained the reason why the deadline was extended.
Try it: Choose the natural sentence. (The reason why the room changed was simple. / The reason why of the room changed. / The reason why because the room changed.) Answer: The reason why the room changed was simple. — 'The reason why' is followed by a full clause (the room changed) and then a main verb for the whole sentence (was simple). Adding 'of' or 'because' after 'the reason why' repeats the reason word and breaks the structure.