The in-universe answer is that technical difficulties with the visor-masks caused stressful visual issues that would lead to accidents and suicide (though how much can be directly attributed to Abe remains to be seen; AO veterans will recall tricking sligs into walking into mines, off ledges and into electric walls by speaking, whistling and farting repeatedly) so a new, more advanced model was released.
The real answer is that the visor masks were not expressive enough for the collective slig character, and the new designs rectified that.
In future, the Oddworld Q&A thread will serve you well for questions such as these.
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