Good question. I'm not sure, but thinking outloud here (or really thinking while typing, and is that dangerous?), a larger wheel definitely absorbs bumps better, all things being equal. Moulton showed that a suspension system can compensate for that. I test rode one of those old bikes, and if I could close my eyes for a while, I suspect I could be convinced it was a large-wheel bike. What is the cost of suspension? Is it expensiver? Trickier to engineer? Or do old habits just die hard and refuse to accept the idea of adults riding small wheels except for special reasons such as needing to fold the bike?