In most cases, "compatible" means that a set has been tested and a template developed, to allow for auto configuration. Many users do not have the technical know-how, nor the time, to manually configure, or develop their own template. While most SIP compatible sets will work with 3CX, for most (basic?) features, some things may not work as expected.
If there is enough demand for particular sets, then there is a good chance, that after testing, they may be added especially if other models from the same supplier, are included.