Because it's on Playstation? The 360 demolished the PS3 in NA throughout the generation, it was also the leader in the UK and was overall a sales juggernaut. 3 great franchises? yet all these franchises together could not beat a GT game. Forza has been bundled a tonne in all regions, given away for free, yet it still can't dent GT.
Gotham and Horizon aren't close to sales juggernauts and now you're mixing sims with arcade racers, so if you want to do that, let's pretend that the PS3 never had Motorstorm 1 and 2, Mod Nation Racers and LBP Karting. Let's also forget that F1 championship 2007 (which was a beast of a sim on PS3 prior to GT) also never happened, and if you're just talking about racing thrills and speed, let's forget about Wipeout HD's existence. If these 2 arcade racers were competing with Forza and hindered it's sales, then the same can be said about the many more arcade racers on PS3.
As for your question, GT5 did have some performance issues due to the hardware (difficult PS3 development), yet GT5/6 was so much more ambitious than Forza 4. Better resolution, better graphics, better effects, better driving model, day night cycle, weather, bpsec and the list goes on....Poliphony are just too ambitious for their own good. So I'm sure if they did a less ambitous racer with less features like forza at 720p, they would have easily locked 60fps with no weather and no day night cycles and perhaps score better too.
Having played Forza, it's much easier to get through, it's an easier game overall, so the appeal to more casual gamers is definitely there, have you seen footage of how some of these reviewers drive a car in these games, they can't even keep it on the road for the most part. I know some people who are having difficulty pulling out bronze trophies in some GT challenges and licenses to this day, so I guess all of this combined helps forza, but I'm not sure I'd want it any other way. Let's be fair here though, if you're pumping so many forza games per generation, accessibility is definitely something you want to be looser, if not you're only going to impede F5 players running to buy F6.