If you were to force all the Deaf people of the world to use the same language, Deaf people would still communicate with their local communities far more often than communities in other countries, which means that neologisms and accents and so forth would gradually develop and accumulate, until there would eventually once again be hundreds of different regional sign languages — unless you were also continually suppressing Deaf people's attempts to control their own languages, which is both a terrible thing to do and also not particularly sustainable in the long run.

...So by imposing a "Universal Sign Language", you would've really just eradicated an enormous source of understudied language diversity and global cultural heritage, in order to not-solve a non-issue. Deaf people already have ways to communicate across language boundaries that they're perfectly satisfied with, including, among others, the international auxiliary pidgin language used in the above video.