Thank you for the opportunity to help you with your question!
Habitat: organisms in different areas cannot mate since they dont meet.
the selection of a possible mate, individuals from different species may
discard each other as they don’t have the same mating rituals.
individuals from different species may be in season at different times
in the year so they won’t be interested in copulating at the same time.
Mechanical: different species may have different sex organs which aren’t compatible. It just won’t fit.
fertilisation to occur, the sperm must reach the ovum. For this to
happen they usually attract each other through chemical means, but the
chemicals might vary from one specie to another. In this case gametes
won’t recognise each other and fertilization won’t take place.
Please let me know if you need any clarification. I'm always happy to answer your questions.