I get this question from a lot of men that have prostate cancer.
Although the black label warning is a concern to some doctors, and has been discussed here in previous posts, I think the main reason is simply that most doctors don't know about it.
Here is the single best paper reviewing the three major trials that compared Dutasteride to placebo for men with PCa. They draw the conclusions that:
"While the analysis had several limitations, the results showed that dutasteride reduced the risk of detectable prostate cancer, including low-grade prostate cancers, which are less likely to lead to death. Additionally, in our analysis, there was no increased risk of detectable high-grade prostate tumours, defined by Gleason ≥7 scores, as shown in CombAT and REDUCE [trials]."