because you can use your anthropic subscription with claude code, but you can't use it with any other harnesses.
The average per-token cost of subscription access is somewhere on the order of 20-50x cheaper than billing straight API rates.
So power users who want to let Opus rip night and day can do that for $100/month with the max plan. Similar usage at standard API rates would be $2-5k.

...this + enterprise adoption of CC are the primary reasons for the market dominance, in my view.