I’d contend that the notion of revealing itself to the world in prior decades would have seemed suicidal.
We can infer from the source material that Wakanda would have to be decades, perhaps even centuries ahead of the rest of the world for most of its history- for instance, the first spy satellites are more than a half century old, so it would have to have had cloaking technology for its cities since at least then. It follows that their defensive abilities must have been similarly far ahead for most of this time.
That said, a look at the geography of the MCU version of Africa suggests that Wakanda is a fairly small nation. It isn’t unbelievable that a smallish nation with a limited population might not want to squander precious lives deterring much larger, more populous, if more primitive nations. It’s also suggested that it has natural geographical barriers that restrict migration across its borders, but I’ll get to that in a second.
Also consider what the word deterrence means in this context. The entire concept of deterrence is a fairly new concept: it implies a certain level of sophistication in international institutions and mass media. It’s largely (but not entirely) a doctrine of the atomic age, when nations couldn’t just casually embark on adventurous wars to expand their territories, as the European nations did for much of the last thousand years.
So the possibility of “deterrence” has only really been on the cards for maybe a century or so, and really only the last few decades (bluntly, the feudal era is out). And what has the rest of the world been doing during this era?
Well, there’s been the colonial era (when the populations around them were being conquered, enslaved, stripped of people and resources and client states installed) from four centuries ago until roughly seventy years ago, when two massive planetary conflicts ravaged the world. Following from this, a Cold War where two massive nuclear armed alliances threatened to blow up the world (have a thought about what knowledge of Wakanda would have contributed to that situation). After that, the world is then dominated by a single nuclear-armed superpower with a history of overthrowing uncooperative nations and a notorious reputation regarding its treatment of people of colour. It’s also the only nation to ever use its nuclear weapons in a conflict.
You’re the ruler of Wakanda at any point until recently, what do you do? Your population is relatively small, but is safely out of sight. Remember, if you openly venture beyond the natural geographic borders that enclose your kingdom to help your neighbours, you’re committed- there’s no way back from that. At what point would that not look incredibly dangerous? At what point, as Steve points out, would that not have virtually committed them to their own imperialism out of sheer survival?
Frankly, if I was T’Challa, I’d have been tempted to have kept my mouth shut.