Sun stall is used on occasion—the biggest issue is that sun doesn't really have 'abusers' in the same way that hail or sand does. Sure scaf/band victini and darm are cool in theory, but they're more on par with kyurem + hail in terms of threat level. The real abusers (reun on sand/hail, zam on sand, thundy on rain) are what give weathers their distinct advantage. For the most part, sun's abusers do not possess that same threat level.
The other issue with sun is defensive. HO is the only other style who has similar defensive issues to sun ofensive, but it at least has the idea of mitigating the issue by immediately pressuring the opponent t with a barage of setup sweepers (SD loom, yache chomp, scizor + DD nite). The idea is that common checks (scarf keld, lando), will be worn down by the constant barrage of offense. Sun simply...isn't able to play that fast. From the get-go, there is tension between needing to get your weather up or bouncing (Xatu) / spinning hazards (forri), but the bigger issue is speed. Sun simply does not have the level of longevity needed such that it can switch around and apply support from stuff like tran or ninetales—you need to be constantly on the offensive. But you also aren't exactly outspending stuff because the fastest you go is scarf latios, and you need to preserve that to a degree so that you can actually check special attackers. You could go scarf on one of tran / victini / darm, but then you're still outsped by keld and surf has odds to 2HKO tini in sun, and darm has a 31.3% chance to fold outright to a single pump in sun. That's not to mention how horrendous it to pivot your scarfer in against Zam, who actually can switch moves. Spdef heatran doesn't really solve the issue—rain still shreds you apart.
Again sun definitely isn't dead, and there's been some cool variants where tales is used primarily to remove sand and enable a Volc sweep. At the end of the day though, sun in current BW OU is high-risk low-reward. It's biggest 'positive' matchup, sand, is currently in its most magic-guard / defense-spam phase it's been in for a while. Rain still trucks it, and you are terrified of HO terrak and ttnite with how hard rocks are to get up. There are (generally) better options, unless you are teccing a sun team for a specific matchup.
