Because the patient will vomit and aspirate. The 30 ml rule is for adults by the way. The gastric mucosa needs time to undergo peristalsis and propel the chyme through the GI tract. A residual of 30 ml means they are full and need more time to empty the stomach. Let's say you went out to dinner with friends and you sat and ate and sat and ate. Eventually, if you didn't stop eating, you would vomit. You didn't give your GI tract time to process what you had ingested. Not to mention, patients receiving tube feedings are in the bed, not walking around like you and I, this further slows down gastric motility and gastric emptying.
The reasons vary depending on the patient. For example, 30cc is a small amount for an adult, but may be too much for an infant; the patient may have had stomach or intestinal surgery, etc. In general, when nasogastric feeding, care must be exercised as too much residual may cause the patient to choke and aspirate.