I think it's the combination of the 30-30, and the Winchester Model 94 that has the 30-30 continue to be popular. For me, it's all nostalgia. The 30-30 is a mediocre cartridge at best, by today's standards. 100 years ago, it was good. My son has a Marlin 336 in 30-30; it's his number 3 back up rifle, behind a 7x57 and a 7mm Mag.
However, when I was in my teens, the dream of having a wild west Model 94 in 30-30 was HUGE. As long as that image of a frontiersman with his Model 94 in 30-30 remains, the 30-30 will endure. Young men in their early years, with a huge passion for being frontiersmen, will buy the 30-30 in a Model 94.
And, let's be honest. The 30-30 will kill a Deer at a reasonable distance. My son has killed Deer with his. I never owned one, so I haven't. I went straight to a Remington Model 722 in 300 Savage when I was 15 in 1949, bypassing the 30-30, all because my Step Dad lit that path for me, because he knew a bolt action rifle in 300 Savage was better than a lever in 30-30.
But, I expect the 30-30 will be hanging around a lot longer.
Martyn