You might be new so here is the quick answer: Because people have been talking about it for 3 years and it hasn't happened... Asic Manufacturers quit waiting around
Been mining for about 2 years now and while it has been delayed for so many times, I feel like this time around POS seems more likely to happen. With all the developments and activity going on in the Ethereum github page, pretty soon all mining may come to an end.
The E3 may probably be the last ASIC i buy. The 11 series GPUs may turn out to be a good investment if anything, for resale value.
Casper will not be launch before 2019 so POS....