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I imagine it's a simple time thing. Notice how TeX, in the footer, comes after all of the sites currently on Data Explorer?
TeX was launched in November. The last revision of Data Explorer stops on October 30th. It's been previously mentioned that only graduated sites will be measured on Data Explorer, and TeX simply hadn't graduated by the time the Data Explorer was updating.
Now... I'm noting that Stats did actually launch after October as well. Stats on 11/4, and TeX on 11/11. I would believe this to be because the Data Dump was done on November 10th, which places it after Stats but before TeX. Hence why Stats got in but TeX didn't.
Note that the above mostly determines whether a site is included in the data-dump. However, Seasoned Advice and Mathematics are not actually on Data Explorer, despite being in the data-dump (and the latter being one of the largest, to boot). So while time does play a big factor, there's also some other (currently unknown) factors that also figure in.