It seems that the "older?" i7-9XX series processors are still quite common in new systems. Is there an advantage to these CPUs over the newer Sandy Bridge i7-2XXX series that is making them preferable?
Advantage-wise, really the only advantage to the older i7 is cost. In general, especially in the bulk volume that a vendor would buy, they are slightly cheaper than the Sandy Bridge line. As with any corporation, companies look to cut every bit from their costs wherever they can.
Another potential reason you may see the older line more is that the vendors may have large existing stocks that they are still working through.