Here's Why The World Can Never Have One Universal Language
To answer this question we need to consider why we have multiple languages in the first place.
Presumably at some point about 100,000-200,000 years ago, homo sapiens started using language in the way we mean language now. At the time, we were spread out over a relatively confined space on the globe, and it is practically impossible that language spontaneously arose in more than a handful of places.
So, at one point, there were some limited number of languages among groups of people that had some amount of geographic proximity. It could have been relatively easy for one language to emerge then. Everyone could have spoken the same mother tongue, and things could have stayed that way until now.
But that didn't happen. In fact, the opposite happened. As humans spread across the globe and population growth exploded exponentially, so too did the number of languages. In fact, it's estimated that there were approximately 10,000 languages spoken only a couple of hundred years ago.
There are two reasons for this. The first is that languages change. The second is that language is identity.
It's easy to see that languages change. Remember struggling through Shakespeare? Yeah, me too, and that demonstrates the change that English has undergone over the past few hundred years. When that continues to happen and the same language changes in different ways in different geographical regions, you eventually get new languages. The most obvious example is Vulgar Latin dialects turning into the Romance Languages - from one language to many.
So, the first part of the answer is that the general tendency is for languages to propagate and diverge.
Your response may be that now, we are in a new world order with globalization homogenizing the entire world into one common culture, facilitated by internet technology: America's melting pot writ large.
This is where part two of the answer comes in. Language is not simply a means for communicating. Language is also identity. We know that people communicate more than ideas with their language. Subconsciously, they also communicate who they are, what they believe, and where they're from. So the obstacles to one language are similar to the obstacle to us all wearing the same clothes.
It would certainly be cheaper and more efficient to have one language, but it's not how people behave. And we see that empirically in studies of how Americans' accents have not homogenized with the advent of TV.
The same applies with languages - in the face of globalization, we see renewed interest in native languages, like the rise of Gaelic (an Irish language) in the face of the EU.
Colonialism and statism have led to a decline in the number of languages from its peak of 10,000 to about 6,000 today. So long as countries exist, English won't encroach further. In other words, the world doesn't really want a universal language.
Humans aspire to have their own distinct identities and form different groups. The same aspirations that drive us to wave different flags, root for different teams, listen to different music and have different cultures mean we'll continue to have different languages.