esbuild is and has always been governed strongly by Evan Wallace. I can't vouch for whether or not the Evans (both Wallace and You) talked to each other about it, but I'm almost 100% certain Wallaces' vision for the esbuild project is different than what rolldown is ultimately trying to achieve.
In some respects, rolldown is also the next generation of Rollup[0] and is a direct answer to rspack[1], which is a webpack compatible rust based toolchain from ByteDance
[0]: Which switched to SWC as its underlying parser from acorn, which makes me wonder if that influenced the decision to use OXC, perhaps they found SWC to be troublesome. I know the Rollup and Vite teams are extremely close, and wouldn't be shocked if any troubles they had with SWC weren't shared with the Vite team when considering underlying technologies