Because the 480/580 have been out for over two years, at a low price point. They offered similar performance to the 290/290x, which were both available for 480 prices back in 2014, and let us not forget the 390/390x, where were available for as little as $200 brand new, that offer the same performance.
At this price/performance bracket, the 590 is offering the same thing AMD has been offering for FOUR YEARS now.
Hey, genius, where is your evidence that people couldnt get their hands on these cards? The 480/580 and 470/570 sold quite well against nvidia initially. AMD and nvidia doesnt see these drops "a few months after launch" because you cant saturate the market after 3 months. You CAN saturate the market over the course of several years, which is what AMD has done. The 590 is an interesting offer for gamers still using old 7800 or 7700 series cards. Go look at steam's hardware survey, that is a TINY number of users compared to 480/580 owners or, god forbid, pascal users.
You seem to think that mid range buyers never bought 480s or 580s. While mining did jack prices up, there were plenty available at launch prices for months beforehand, and the used market has been a good source of below MSRP 480s for some time now. The price bracket also hosted hawaii for some time, which offered 480 performance at 480 prices for many months after the first mining crash. You have had 480 level performance for 4 years in this segment, and for some reason you think there is this huge group of PC gamers that STILL havent upgraded, but will open their wallets up for a minor 580 revision because ???
News flash, the people in the $200-300 bracket do not upgrade for minor performance increases, they only buy for massive gains in the same bracket, like 7850-480 level. Those people have had ample time to grab cards, and if steam is anything to go by, many have already upgraded. The 590 will not appeal to them, it will appeal to remaining 7800-7700 series users, which make up a small minority of AMD users today.