There are languages created by groups, but they aren't international and they're more like dialects of existing languages. For example, Mandarin, New Norwegian, and Modern Hebrew; these are all successful constructed languages (or dialects) but I doubt they were made by a single person. You can also argue that languages like French or Icelandic, which almost always create all new words instead of taking loanwords for new ideas and things, are artificial (this is done by groups and organizations too).
But for a world-wide language project, the people who want to create a good international language have no money, therefore no support, advertising, clout, etc. The existing organizations, like the UN, which by all means should create their own language or use some kind of bridge language like Esperanto, don't want to do so because it means "change" (they're scared of changing stuff and then failing even more than they are now, on the language issue). For example, ANY language no matter if constructed or not, would get big if it sent abroad as many movies as the USA does; likewise, anyone would try learning a language if they were given a fistful of cash in order to do so (meaning, learn it and we'll pay you extra to act in a movie for it).
However the people who do have this kind of money don't care one bit, for example, many rich Americans donate extremely little money to charity compared to, for example, rich Swedes - and creating or sponsoring an international language is just like a form of charity. Otherwise, organizations actually look for solutions in the wrong area without even considering the constructed language idea, ex. simply telling everyone to learn whatever is the biggest language at the time (French, Latin, English, Chinese), simply spending more money on translators, simply trying to have kids learn 3 foreign languages in school, etc.
The last aspect is English itself, basically English-speakers are much less likely to learn other languages and they're much more likely to force their language on others (McDonald's advertises in English here in Sweden, for example). This is true whether it's the UK or the US, but the problem is that those countries are where a LOT of money is, a lot of organizations, and so on - and they live in such an English-speaking bubble that they don't actually realize how much of a language problem exists for the rest of the world. The basic idea that most countries nowadays have is, if the English-speaking population doesn't like something, it's not going to get enough money or support for it to become a world-wide success (which isn't true at all, but it's the psychology that matters).