I really like Ezra Klein's analysis of the Cruz, Paul, Sanders, and Wendy Davis filibusters/extended speeches, trying to answer the question: why don't more Senators pull this stunt, since it seemed to work so well for the four of them? Klein has three answers: doing it is difficult and carries risks (of saying something foolish); it's usually unnecessary in purely legislative terms, at least in the current Senate; and it can be annoying and counterproductive.
All correct under current Senate rules and procedures.
It's not surprising, then, that the three Senators who have done it recently share quite a bit in common. They're all ideological outliers; Paul and Cruz probably couldn't do much legislatively even if they tried, and Sanders isn't quite in the mainstream of Senate Democrats, although by nature he seems far more interested in going along to get along than Paul and Cruz do.
At least Cruz and Sanders are also good talkers, politicians who definitely have the ability to make it up as they go along (I know less about Paul, and even less about Davis). Not that it's strictly necessary; at least if rules allow it, as they do in the Senate, extended speeches can consist entirely of reading written material, and as all of these cases proved the world will provide plenty of material for any filibustering politician to read once he or she gets a sufficient amount of attention. I'd add to this that arrogance is probably a plus: not just that they have the ability to talk off the cuff at length, but that they believe that they can do so and that they can change opinion with their efforts.
Moreover, it's no surprise that at least two of the examples were seeking higher office (I don't actually know whether Davis was already intending to do so, but I wouldn't be surprised).
What all this suggests is not only, as Klein says, are there disincentives for these long speeches, but they most likely work together to severely limit the number of Senators who are interested in it. Still, I wouldn't be at all surprised if there are more of these in the future. Probably right up to the point where one of them implodes.
Also: nice catch!