I'm just having trouble understanding this. I know that often marine fish are osmoconformers, and freshwater (like bony fish) are often osmoregulators, but why?
There are virtually no freshwater osmoconformers but there are some exceptions like being both an osmoconformer and an osmoregulator based on the salinity of the environment.
Osmoconformers keep their internal fluids isotonic to their environment and they maintain an internal salinity similar to the surrounding water. The concentration of solutes in freshwater is very low and if any organism were to have that low concentration of solutes in its body, it would not have enough proteins, ions, etc. to effectively conduct the biochemistry of living.
Here is a rare example of a freshwater species Daphnia which can be both an osmoconformer and an osmoregulator:
For freshwater osmoconformer species, most of the time there is a gradient between their internal osmotic concentration and that of the external environment; consequently, osmoconformers will vary in their cellular osmotic volume contents and as a result, some FW species (e.g. Daphnia), osmoregulate at low salinity levels but are osmoconformers at raised salinities. Some physiological processes, including reproduction and metabolic rate however, can be affected negatively by these changes (this group regulate their osmotic pressure actively) (Chen and Stillman, 2012).
Additionally, there is the term euryhaline used for the aquatic organisms who are able to tolerate a wide range of salinity. There are fish species that can live in salt water, fresh water and brackish water. You can see a list in the Wikipedia/Euryhaline Article.