According to the film's official novelisation, the Mogwai were the result of a genetic experiment that went badly wrong. Only one in a thousand Mogwai retained their sweet and innocent disposition.
Soon after these first departures it was discovered that Mogturmen’s creatures were highly unstable. To be exact, fewer than one in a thousand retained the sweet disposition and charitable aims built into it by the inventor. Instead, something went wrong. Very wrong. The Mogwai himself knew of the unstable Mogwai, being well versed in the historical background of his species. He preferred not to think of the complications that had developed, but it was nearly impossible not to. It was, after all, part of his heritage. Closing his eyes as he relaxed in his cage awaiting his supper, he mused briefly on the wars, landslides, and famines that had taken place on Kelm-6, Clinpf-A, and even here on Earth because of his creator’s miscalculations and willingness to disseminate an untested creature. Small wonder Mogturmen had been punished by having his . . .
The Mogwai pushed the thought from his mind. True, Mogturmen had failed in the overall, but he himself was one of the successes, the one in a thousand who still embodied all the good things put there by his high-minded inventor. Yet his existence, he knew, had no long-term benefits for society. Gentle as he was, he was a distinct threat to those around him. Just a few drops of water, a morsel of food at the wrong time, and—
You may wish to note that Gizmo isn't the only "minority Mogwai" on Earth. There are apparently three others.
He had seen it happen before, as had the three other minority Mogwai on this planet, and the results were nearly always disastrous. The most recent episode, not caused by a spawn of Gizmo, had occurred late in 1983, when a single Mogwai somehow got aboard an American space shuttle craft, Columbia. Because of strict government secrecy, details were never published concerning exactly how the Mogwai was allowed to reproduce, feed after midnight, and turn into a Gremlin. In any event, the Gremlin eluded capture by the six-man crew long enough to shut down the computer handling the craft’s guidance and navigation systems. When the scientists switched to the number two computer, the Gremlin found a way to cause an overload. It then got into the system that senses the ship’s acceleration, position, and angle of attack. Over the Indian Ocean, Columbia actually started falling out of orbit and was out of contact with Mission Control for forty-five minutes. During that hectic time the pilots and scientists managed to pursue the Gremlin into a storage compartment and kill it. Returning to earth eight hours late as a result of the Gremlin’s meddling, the crewmen were debriefed by government officials, who warned them not to describe what had actually happened on the mission.