1. Heatran is easy to put into KO range from CC, because it has no form of recovery outside Lefties (unless you wanna RestTalk, lol). Furthermore, +2 CC is a clean KO, and so is CC + Return
2. You 2HKO Skarm more easily with +2 CC and Skarm isn't gonna be able to Roost on you because it's lost too much HP and Roost will let CC OHKO (+2 Return is less certain) 3. Except even CB Azu isn't going to OHKO you with Aqua Jet (and CB Azu is kinda exploitable), while +2 Return is a clean KO. Mega Scizor is in the same boat. Sure, they deal upwards of 50%, but seriously, if you're gonna use Pinsir, get the rocks off first. Pinsir isn't usually prio weak save for Talonflame (which will be a problem whether you run CC or not) so I don't see this being an issue.
In addition, Pinsir also unconditionally crushes Tyranitar, which was once a solid check to Bird Spam, with CC. You also don't have to worry about Air Balloon Exca, Magnezone, and Zongers, all good Pokemon on the rise, getting free switches into you. EQ is nice, but it is far more exploitable than CC, and is usually a bit weaker except for some targets (note: Heatran) that are dented hard by CC anyway.
Flying/Ground coverage is nice, but face it, Flying/Fighting coverage is godly. Pinsir doesn't have to run CC, admittedly, but the fact that it can more realistically choose to run CC over EQ just made many more of its checks cry and allows it to break through things more easily. That alone just made the task of checking Pinsir a lot more daunting, because many things that could check Pinsir once now need to tread carefully or risk getting flattened by CC. Maybe running CC over EQ alone might not push it to S rank, but the fact that it is a much more realistic option with Aegis gone is a pretty big deal. Oh, speaking of Aegis being gone, Latios, the premier defogger for offensive teams, got a hell of a lot better, and you got Starmie too as a reliable spin option. Pinsir is a huge meta-defining force right now, and even when you know what it's gonna do, it still puts a lot of pressure because it does its one set so well. Sure, it needs support, but Defog/Spin support is a given for many teams and is a bit easier now that Aegis is gone and Bisharp is falling out of favor. Simply put, things got so much better for Pinsir with the Aegis ban, and it came from a top-tier threat to a total force to reckon with.