Despite the technical hurdles, there were compelling business reasons:
Technically, there was never an official, full-featured Viber application released specifically for the generic Java J2ME platform. Viber focused its development on operating systems with more robust background processing and VoIP capabilities, such as Symbian, BlackBerry OS, and Windows Phone. Viber For Java J2me
For historical accuracy, here is how users would have installed Viber on a Java phone ca. 2013: Despite the technical hurdles