Steel frames fail because of road vibration resonance that slowly tears a steel frame apart where the vibration is stopped, i.e. the bottom bracket. Thicker walled steel tubing will stop some of this breakage by being better able to sustain/absorb the road vibrations

http://www.brighthub.com/engineering...es/111822.aspx

"The nature of resonant vibration and the accompanying amplification of fatigue stress are discussed in relationship to the damping energy absorbed by a vibrating system. The resonance amplification factor is defined as a measure of the severity of a resonant condition. The sources of damping in a
vibrating system are discussed and classified according to whether they are external (structural) or internal (material). Data on the internal damping properties of a variety of structural materials are presented and the generalizedbehavior is discussed. In cases where internal damping is significant, the
importance of both fatigue strength and damping properties of materials as joint criteria for resonant strength is demonstrated and quantitatively expressed. The analyses are made in terms of the resonant strength constant for the material (the material factor) and the volume-stress function of the part (the part factor).

http://www.dtic.mil/dtic/tr/fulltext/u2/039439.pdf