The S&P said Thursday that it's not changing its rules to fast-track mega-cap IPOs onto its marquee stock index, the S&P 500.Why it matters: S&P's decision doesn't mention SpaceX, but it means that Elon Musk's company — as well as Anthropic and OpenAI, if they go public as expected — won't be making it into the index for at least a year.And it's a surprise, coming after other major indexes drew fire for changing their rules to more quickly accommodate Elon Musk's company.The big picture: Millions of passive investors won't be forced to own SpaceX quickly — a move many had criticized.Between the lines: If the SpaceX IPO, coming next week, pops — that may set the S&P up for criticism. If it flops, they'll look smart."We'll see whether this turns out to be a wise choice," Bloomberg Intelligence senior ETF analyst Eric Balchunas posted on X. "Either way tho SPX is such a powerful brand it prob doesn't matter either way."Zoom in: Under the rules, companies must wait a 12-month seasoning period before being included in the cap-weighted S&P 500 index. The group considered changing that to six months, as well as updating other requirements.It also considered adjusting the so-called "float requirement," which held that a company must offer at least 10% of its overall shares to be considered for inclusion. SpaceX is planning to offer fewer than 5% of its shares.What they're saying: In a statement, S&P said the decision "preserves core index principles.""Although there may be trade-offs between strict adherence to these eligibility requirements and broad representativeness, the current methodology provides substantial market coverage and sector balance. As a result, the indices can continue to meet their stated objectives while preserving their role as representative and investable benchmarks for the U.S. equity market."Yes, but: The S&P did change some rules for some of its broader indexes —including the S&P Total Market Index and Dow Jones U.S. Total Stock Market Index.These are meant to capture the entire breadth of the stock market, compared with the S&P 500, which covers the very largest American companies. For those broader indexes, S&P came up with an alternative float requirement for very large companies that are offering less than 10% of their shares.Go deeper: SpaceX and the coming stock tsunami