The game needs to have felt updated in some way. Take Final Fantasy X HD. I remember how slidey all of the characters' movements were and that was kept intact, but it made the game feel dated. Yes, graphics are a big deal, but you also need to update animations. Then you also have completely botched games like Silent Hill HD Remix which lost a lot of the atmosphere of the original.
Then there is the convenience factor that was mentioned. It needs to make the game simpler to play. The thing about Last of Us Remastered is that a lot of people played on 360 only and did not get a PS3. Sony a few months ago said that over 30% of PS4 owners did not have a PS3 and my guess is that this is based on new PSN accounts on PS4s. Can it be called a cash grab, sure but when there are a bunch of your user base quite vocal about a title you can't exactly get made when they make it. Sure you may not like it, but there were a lot of people who said, "I want to play this game, but I don't want a PS3." my PS3 was on the brink of death when Last of Us came out and for me the experience was unplayable. little did I know that the month after I picked up the game my PS3 died (it was a launch model, so I think it did pretty well). So I welcome the remastered version.
On top of that, backwards compatibility is kinda tricky. Sony's PS3 games were designed to work with the Cell processor which doesn't work the same way other multi core systems do. With typical multi-core systems everything takes a new chunk to process. With Cell, everything had to go through a manager core who was distributing the work among smaller cores and the only way the manager core would know what to do is if the designers told the manager core which smaller core to send a particular packet of work to. Because not a lot of devs cared to put that extra work into it, the PS3 was effectively a single core system just letting the manager core do everything.
Backwards compatibility is also costly because that is hardware. 360 had limited backwards compatibility which meant only first party Xbox games could work on 360 and even then, not all of them. PS3 had 99% backwards compatibility, but that pushed the price tag to $200 over the competitor. Who would buy a PS3 at $599 when the competitor who played games better and was the lead platform at $399? That is the cost of backwards compatibility. Sony saw sales numbers were not great, took out backwards compatibility, dropped the price to be competitive with the competition and saw sales take a positive turn. Sony saw people vote with their wallets. Sony heard "we don't want backwards compatibility, we want a less expensive console." That is what we got with PS4. I think PS3 was holding onto PS2 backwards compatibility for 2.5 years before they finally dropped it.