If they accept Brad Katsuyama, there must be some other reason for excluding Stiglitz than criticising HFTs. Katsuyama has not only done the same very publicly but literally created a company to build a trading platform that nullifies HFT strategies by design.
Katsuyama created a trading platform that nullifies one HFT strategy, namely latency arbitrage.
Stieglitz is much more critical of the strong efficient market hypothesis in general; from this, he derives a number of positions that are anathema to the entire trading community - among other things, he has advocated a Tobin tax.
In short: Katsuyama is someone that will help with technical tweaks to get rid of the most egregious abuses of HFT (which would benefit the other players at the table, e.g. the trading organisations and exchanges), Stieglitz would probably advocate higher transaction costs (which would decrease overall trading volume and hurt the other players at the table).
So Stieglitz isn't invited because he's seen as "bad for business".