Why did Scrum become synonymous with Agile?

Ah yes, the Great questions in life... I would guess that at least for popularity, it is a very good framework which maximizes freedom of implementation using the bare minimum of "rules". So, any company, department or even team can implement it as they see fit. Its like a swiss army knife, small and compact, but you can do anything and everything with it. Also, it can easily incorporate other techniques, frameworks, tools and ways of working like XP. It is an inclusive framework. So it is a universal container, a recipe which will give you a great meal, but which allows you to add ingredients if you like, on the go.

Some point why I think it is so popular, and because of popularity, the reason why it is becoming more synonymous.

As I recall, in the early 2000’s XP had the greatest mindshare amongst developers. This was seen as a reason for management not to support it.

Scrum appeared less challenging to managers. They did not understand it or any of the change that would be involved, but the name was memorable.

Why did the iPhone become so popular?  Why not the Blackberry?  Why did SAFe become so popular?  Why not Scrum@Scale if Scrum is so popular?  I don't think anyone can answer your question. As @Ian said, it could be because managers saw it as less threatening or it could be that developers decided that XP was too strict.  

Humans are strange.

I agree with all the above. What I've learned, though, is that you can be Agile without Scrum and you can do well with Scrum without being really Agile. 

I do think Scrum is the perfect place to start, if you want to become Agile. Everything about it supports agility. And as Ian mentions, it's more manager-friendly than XP, because it's 'well defined' (although hard as hell). 

And the same to Daniel's remark -SAFe over scrum@scale- or LeSS/Nexus etc.... Even though we're way past the Early Adopter stage with Agile, managers still feel better when they can maintain a bit of a subjective-security-blanket by investing in something they can more closely relate to. And that brings me to another thing I really learned in my SAFe training 3 weeks ago: it doesn't matter which framework you decide upon, or even framework vs. #noframework (do it yourself). If you don't have the right people, with the right mindset in place, it ain't gonna work. Even SAFe can be Agile as hell, with the right leaders.

I do think Scrum is the perfect place to start, if you want to become Agile. Everything about it supports agility. And as Ian mentions, it's more manager-friendly than XP, because it's 'well defined' (although hard as hell).

@Robert Shandley, Why not Kanban? Kanban is less prescriptive than Scrum and much more easier to start. Any reason why Kanban would not support Agility in comparison to Scrum?

@Daniel Wilhite, SAFe's popularity in my opinion stems from the fact that it is quite prescriptive yet gives opportunities for flexibility.

@Steve, I tend to over-focus and over-simplify. I gave an opinion to the original question, without regarding the whole question. I am however an experienced Scrum Master and have limited (theoretical) experience with Kanban etc... so I tried to express a qualified opinion about that which I know. As a side-note, I thank you for the chance to explain myself, or even better, own up to a F**K Up;-)

But back to work: in my relatively short (but intense) term as Scrum Master and Agilist, I've come to the hypothesis, that managers are more comfortable, the more well-defined and prescriptive a framework or method is. It's the natural way of thinking. Scrum is prescriptive. Not immutable, you do it or don't call it Scrum. SAFe is the same. You do at least Essential SAFe or don't call it SAFe.

So why is Scrum synonymous with Agile? And on an aside: why is SAFe the most popular Agile scaling-framework?

Just an opinion.