You cannot compare Doctor Who to political thrillers aimed at adults, nor even to sci-fi programs like Stargate aimed at adults. If Stargate can be compared to anything, it's Torchwood, but it's certainly not Doctor Who. Again, one is aimed at families and one at adults.
Really, can Doctor Who be compared to any other series on TV in the U.S., the U.K., or any other part of the world? I thought part of the reason why Doctor Who has always stood out is because it's such a unique animal that can only be compared to itself or, in rare cases, its own spin-offs like K-9 & Company or The Sarah Jane Adventures.
To my knowledge, Doctor Who, unlike many other sci-fi series, has not spawned a wave of imitators or informed an entire subgenre. It's not like Star Trek, which established itself as the definitive military spaceship series that shows like Babylon 5, Battlestar Galactica, & Stargate aspire to/react against (or even some of the non-military spaceship shows like Farscape & Firefly). Nor does Doctor Who fall neatly into a subgenre like paranormal investigations (i.e. Fringe, The Chronicle, Primeval, Sanctuary, Special Unit 2, Torchwood, The X-Files, going all the way back to Kolchak the Night Stalker) or paranormal anthology (i.e. Night Gallery, The Outer Limits, The Twilight Zone). I'm not sure we can thoroughly explain its lack of popularity in the U.S. until we can adequately explain its singular longevity in the U.K.