It is mostly because of economic reasons. The post-top ones are cheaper than those with outreach arms. Another reason is that some cities find that post-top street lights can make their cities look more beautiful. As for steel cables, nowadays people try to avoid overhead power lines. Although underground wiring is costly, overhead power lines have too many disadvantages, such as maintenance, safety, appearance, etc.
More reading: Height of Street Lights: How Tall Should They Be?