Between the two world wars, for example, sterling and the dollar were both active as international currencies. This period, of course, was clearly transitional, and the dollar subsequently became dominant. But the most important factor inhibiting the emergence and persistence of a single vehicle currency throughout the world is the attraction of portfolio diversification.
This can be a powerful counterforce, especially because currencies offer far greater opportunities for diversification than most other assets. The average price of all currencies, by construction, is trendless, tending to increase the negative covariance within a portfolio of currencies. In contrast, equity instruments are often driven in the same direction, as are debt instruments; and often debt and equity prices move together.