A hypothesis based on the accretion model of planet formation is proposed which can explain why minor planets could not grow into full-sized planets. Temperature and impact-velocity conditions for bouncing, accumulation, and fragmentation of planetesimals are evaluated for silicate and iron planetesimals at the heliocentric distances of Mercury, Venus, earth, Mars, and the asteroid belt. The results indicate that the low-temperature environment of the asteroids very likely hampered their growth into ironlike planetesimals, which in turn inhibited the accumulation of silicate planetesimals. It is concluded that the brittleness of iron at temperatures below 200 to 250 K is the primary reason why minor planets are 'minor' and could not grow to full size.