Partially it was that stainless steel was cheaper. A bigger issue was that making large carbon fiber structures takes much larger than with steel and so it would really have eaten into their iteration time. But also while the strength to weight savings from carbon fiber are a big deal at regular temperatures the heating from Starship reentry erased that.

And they abandoned it to try to eliminate the need for a heat shield. This plan did not pan out.

The whole point of a reusable launch system is the cost of the vehicle is amortized over many launches, so you can use expensive, high performance materials.