Maybe because the Boss was a much more significant car than the Mach 1. They made 1600 '69 Boss 302s and 7000 in '70. For comparison Mach 1s are everywhere, they made more the 72,000 in '69 and another 40,000+ in '70. The Boss gets more attention because of the motor, suspension, rarity. The Mach 1 is a nice car, but not it isn't on the Boss 302's level.
Yes, they're nice, but the 302 is overrated. I do agree about the 429, though. They are much better in terms of performance. But what about the 428 CJ Mach 1? You can't those aren't not cool, not 429 level, but a bit above the 302.