It's unfair to those speaking a different language. There's also the extinction of other languages (and all the work that went into them, and all the works that used them, literature and associated culture). It's like the extinction of species in the rainforest, except for memes (in the original Dawkin sense).

The reason is the network effect, in that the more people who use a communications system, the more valuable it is (given that communication is valuable). Language is a communications system.

This forms a natural monopoly: because of the value of larger networks, it is inevitable that one language will dominate - unless there are other barriers partitioning the network, akin to the lack of a telecommunications physical layer in centuries past.