Sure, the song's co-author -- Matchbox 20's Rob Thomas -- was a relevant hitmaker around that time, but even his band didn't earn their first No. 1 on the Hot 100 until after "Smooth" came along.
So how did "Smooth" ascend to supremacy? Well, in part, 1999 was very amenable to Latin-flavored music on the Hot 100. Ricky Martin scored a five-week No. 1 with "Livin La Vida Loca" earlier that year, and Enrique Iglesias (who although Spanish is considered Latin pop by genre) brought his Spanglish single "Bailamos" to the No. 1 spot for two weeks in September.
Cross-cultural appeal certainly buoyed "Smooth," but its cross-generational element was arguably just as important. Rob Thomas' presence attracted listeners half the age of the Santana demographic, but his impassioned performance on "Smooth" was accessible enough for baby boomers to latch on to. Similarly, the retro Latin rock vibe courted old time Santana fans, but the instrumentation was fresh and vital enough that it didn't sound too old fogey to younger listeners.
Unlike Elton John's "Candle In the Wind 1997," another long-running No. 1 hit from a classic rocker, "Smooth" was a ubiquitous hit, too. Elton's ode to Princess Di didn't pop up at social events in the same way "Smooth" seemed to. You couldn't go anywhere -- restaurant, party, grocery store, Blockbuster Video -- without hearing Thomas spit out "Or else forgettaboutit!" followed by a sonic punch of horns. And of course, it remained on the radio long after Christina Aguilera's "What a Girl Wants" knocked it off the No. 1 spot on Jan. 15, 2000.
While other singles sold more copies in 1999 -- LFO's "Summer Girls," Christina Aguilera's "Genie In a Bottle" -- they didn't have the same cross-cultural, cross-generational impact that "Smooth" did, and still does. Even if you feel like you've heard it dozens of time, check it out again above. It really does stand up.