Since they'd bought them, they had to use them. EK grew at a pace where they could order enough 77Ls. In the case of SQ, they removed P. Eco (though I'm not entirely sure about the reason for doing so), and the all business layout means the revenues are enough to cover the higher operating costs. As for EY, they operate too few routes to be able to do this.

Also keep in mind that because EK squishes in an extra seat in Y on the 777s, the aircraft type brings in more revenue. They can also carry more pax, so the decision may also have been based on higher loads.

I recall reading about this somewhere, but I'm not entirely sure of what all the reasons were.