Why is cromolyn sodium solution for nebulizers no longer available in the usa?

Not profitable. So few people were using it that it became unprofitable for medication manufacturers to produce or pharmacies to stock the product. Leukotriene receptor antagonists like singulair (montelukast) & accolate, while working by different mechanisms, were found to be about as effective & safe but far more convenient to use than cromolyn. They replaced cromolyn as a non-steroidal controller medication.