Beacuse you have got dependent on porn for obtaining arousal.
Not using porn makes it difficult to get aroused and hence sustaining erections can be problematic. Also, pornography tends to distort ones concept and expectations about real normal sex. The result is that one may not find actual sex so intresting. Masturbation problem is very commonly seem along with pornography use.
Next Steps
You must consult with a sexologist in detail. It is treatable. You can ve abke to enjoy normal sex.
Health Tips
It is treatable. Don't worry.