Only because up until recently getting in the HD game was prohibitively expensive due to the engineering and capital requirements for designing spinning disks. Now anyone can buy some flash media, a pre-cooked controller firmware, combine the two and sell at competitive rates. There are something like five or six competitive SSD makers right now and many more bottom feeders. There are two competitive spinning disk makers and its been that way for decades, ignoring the occasional small third-party player like Hitachi.