Why doesn't the executor obtain a subpoena through the probate court?

A power of attorney does not, of itself, create accounting duties - but a breach of fiduciary duty by the holder of power of attorney can be addressed through the courts.

If the executor wants to file a lawsuit, the executor will have to obtain leave from the probate court. The court will presumably want to review and approve the fee agreement. If the attorney's fees are likely to exceed the recovery it may not be a good use of estate funds. Depending on the amounts involved and the policies of the court, it may be possible to litigate on a contingency (percentage of recovery) fee basis.