The most likely answer is because of inconsistent watering. Here are a few guidelines on watering that should help you get a both a good taste as well as keep the watermelon from splitting before harvesting.
In order to get good tasting vine ripened melon, water less frequently but more deeply. Water once a week of one or two inches in depth is recommended. If water is to shallow it may cause the fruit to rot. It is common to water too frequently. Regardless of the weather, there is no benefit to watering daily. The watermelon plant has a deep root system that is resistant to drought and dry climate. Using mulch can also be helpful. Mulch should be added before the melons appear. Reducing water when the fruit is close to harvest and ripening can also help prevent splitting.