It'd guess it was a combination of not being patented (they tried to protect it as a trade secret instead but it leaked), the general ease-of-application of str

It'd guess it was a combination of not being patented (they tried to protect it as a trade secret instead but it leaked), the general ease-of-application of stream ciphers, and the fact that RC4 in particular is so easy to implement you can easily memorise and bash it out in a few minutes.