It's not that Zebras have become unpopular, they are great little birds--funny, active, entertaining, beautiful--one of the best beginner birds. The only drawback is that they tend to be pushy, so you have to be careful if you put them in a mixed species cage. Sometimes people have been successful mixing them with others in a cage, but generally it is just asking for problems, like plucked feathers, etc.
I have my only pair in a cage with a male Green Singing finch--he is pushy, too, so I keep an eye on them, but so far, no problems.