It all has to do with the delta-V and the kind of route you can take to get to Mars. The BFR and its reusability can make it possible to launch from orbit and attain a greater speed as well as not having to rely on Hohmann transfers -- which can require a very long route to Mars (but which are very fuel-efficient, though with low delta-V) taking months there and back.

With the BFR, high delta-V, lots of supplies (and fuel) and other benefits, including cutting down greatly on cosmic radiation and other hazards, we may see Mars travel in a decade, not multiple decades using more traditional routes to the Red Planet. :-)

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