If you mean prewar diecast wheels, it is because of the nature of diecast metal.
Diecast was a cheap metal that most companies switched to from cast iron, during the 1920's and early 30's.
Diecast metal typically has impurities in it that when subjected to temperature and humidity changes over time, expands and contracts.
This expansion/contraction is what causes the cracks, pimpling, warp and eventual failure of the metal.
Weather or not companies knew of this eminent failure is unknown, but it was known that economy was a primary concern. Also diecasting allowed for better details to be produced.
And after all, they were just making toys, not what we would eventually call collector's items.
I recall seeing ads in 1940's vintage Model Builder magazine for replacement belly pans for Lionel 072 streamliners, so diecast failure happened sooner than we may have guessed.
Lionel must have suspected problems with diecast, yet they still trusted this material for their most important Loco, the 5344 / 700E Hudson.
Postwar locos continued to be diecast, yet I do not recall ever seeing a postwar loco that has diecast fatigue. They must have refined the process during WWII.
The main reason Lionel wheels changed to iron, starting 1950, is because it was required to work with Magne-Traction.
