The reason is that fruit is a predigested food that moves through the stomach to the intestines right away - except for bananas, dates and dried fruit. If fruit is eaten with or immediately after another kind of food, it will mix with whatever else is in the stomach and ferment. Let it go through on its own and it will be off dispersing its nutritional values throughout your body while the rest of your meal is busy digesting in the stomach on its own. I found out very quickly that juicing was the best way for me to get a ton of nutrients in a short period of time. I juice fresh fruits of any and all kinds in the morning.
Some favorite combinations are orange and grapefruit, apple and pear, or cantaloupe and kiwi.
In the evening, it's usually a combination of carrots, parsley, celery and broccoli. It's a remarkable way to get a remarkable amount of nutrition. Get all the beta-carotene you can in any way that you can. Of course you can only get what's available in your area and that's just fine. Do some research and find out what is the best for you from your home and juice away.