Vanguard S&P 500 index fund has $227.5B net assets (across all share classes)[1]. Vanguard Extended Market Index fund (which is the set difference of Vanguard Total Stock Market Index Fund and the S&P 500 fund) has $42.8B net assets (across all share classes)[2].
That is 18.8% of funds in Extended Market. The true ratio is 19% of funds in Extended Market[3]. Not so different.
Now you can argue that funds like the SPYder are the ones which receive a disproportionate share of attention. I would counter that (1) SPY is a smaller fund than Vanguard's (2) How much of that is investors vs. traders who use it for the liquidity.
In conclusion, I'm unconvinced that S&P500 index funds receive a disproportionate share of attention in investment, apart from investment professionals discussions. Even Buffet doesn't appear to care, and may simply use the S&P 500 because it is more well-known:
> Okay, so gold is not a screaming buy to Buffett. What should a typical upper-middle-class person in the U.S. buy to prepare for retirement?
> "Equities," Buffett answers without a moment's hesitation.
> "The VTI?" I ask.
> "That's good enough. Maybe a selection of high-dividend-paying stocks that are likely to raise their dividends. Maybe the top 100 dividend payers of the S&P 500."
>Then, after a second's thought, he adds, "Well, maybe not that, but equities."[4]
[1]: https://personal.vanguard.com/us/funds/snapshot?FundId=0040&...
[2]: https://personal.vanguard.com/us/funds/snapshot?FundId=0098&...
[3]: https://www.bogleheads.org/wiki/Approximating_total_stock_ma...
[4]: http://archive.fortune.com/2010/10/18/pf/investing/buffett_b...