Because unless you get a really cheap used ps3, most ps3's cost around $130 on the low end and then you got the two games that can run $20 each. That is over half of the cost of a used ps4.
There may also be other Ps4 titles people want to buy like Bloodborne. Some people may not feel ok spending $150 on a dead system or just want a newer system. Many reasons why people didn't or won't get the ps3 versions and they are all valid.
A big reason to not get a ps3 and the ps3's versions was because 2.8/KH3 won't be on ps3.