This question haunts me as well.
They were like the best selling Android phones from 2010 till 2015. I myself bought one so did many of my family members! A decision we later regretted as they had no software updates and their hardware was of questionable quality. But still they were making big bucks as the general population never bothered about these things.
But I guess when Xiaomi, Oppo, Huawei, Lenovo, Vivo, OnePlus, Honor, Realme, Infinix, Redmi, Tecno happened and flooded the market with their products they hardly stood a chance. Or could be some other reason, i dunno!!