Expectations. It was sold as a next gen, with features that still aren't in the game yet like car modding. The audience basically ended up led to believe it was going to be Rockstar tier and well, we both know GTA5 is significantly beyond it at a technical level, let alone RDR2. It didn't even launch with basic open world city stuff that was established way back in GTA3.

Had expectations been managed differently and people led to expect Witcher 3 but in first person with reasonable gunplay but not top tier then the reaction would've been way different. It's fun for what it is, but you can't get back the fact people wanted and believed it would be something significantly more.