1. Because it’s not possible to make a map for all zoom levels
2. Because the finances are not secured on long term
3. Because the data model is not defined
4. Because the precision is heterogeneous
5. Because the reliability is heterogeneous
6. Because the completeness is heterogeneous
7. Because it requires attribution
8. Because the data are difficult to extract
9. Because noone takes the responsibility about the data
   
10. Because it lacks a QA step by an accountable body
11. Because it is not multilingual
    
12. Because first acquisition is fun and data update is boring
13. Because Google Map Maker Workflow is for the broad public and OpenStreetMap workflow for the map enthusiasts

Having said that, OpenStreetMap is great for rapid maping, has great examples of local success (Best Of OSM) and can be used in a lot of cases.

Two non representative examples: Le Lieu CH or Ragusa IT