Hi, I moved this to a Forum post.
Breast cancer, being a leading cancer in women in terms of mortality (behind lung), is obviously going to have a strong research focus as a result of this. I am not sure that there are many adults who do not know a person who has died as a result of complications arising from breast cancer.
The molecular sub-types that you mentioned were originally defined all the way back in 2000, I believe, by Perou's landmark paper, closely followed by the work of Sørlie:
These molecular sub-types have been consistently verified by multiple other studies and they align well with immunohistochemistry; so, they bring with them a very relevant clinical angle. Others have since attempted to define copy number sub-types of breast cancer (e.g. Curtis / Caldas), but these never quite stuck, possibly due to a lack of concordance with clinical info.
Each cancer is different and, in breast, we can say that these are the true sub-types and that they have been identified and verified by much research. In other cancers, molecular expression sub-types are actually less relevant, for example lung and endometrial.. In lung, we could say that mutation signatures are more relevant as these align better with the clinical data; whereas, in endometrial, somatic copy number and mutation signatures align better [with clinical information].
You also may be surprised to learn, however, that clinical diagnostic / pathology labs are not regularly screening for these molecular sub-types. IHC is still very much the de facto way to screen for a breast tumour sub-type.