1.) His lack of supplies (parrticularly food) and a poor supply chain too far from his base of supply.
2.) If he could take Little Rock which he desperately desired to do would he have been able to have established a reliable supply line to ensure that he could stay there. The naval Battle of Memphis had not occurred by May of 1862 and there was no reliable base of opperations to establish a supply base within reach of Little Rock in May of 1862.
3.) Curtis had no reliable information about the size and composition of the confederates in front of him.. The harrassment operations instituted by Governor Rector and General J.S. Roane in May of 1862 did an admirable job of creating confusion in Curtis command particularlt the 12th Texas Cavalry.
The Battle of Whitney Lane bought time for Rector and Roane. The arrival of General Hindman and the CSS Ponchartrain and Maurepas and their heavy guns in June and the battle of St. Charles dening the needed supplies to Curtis convinced him that the resistance in front of him was stiffening. and his supply situation had grown worst to such an extent that even if he could have taken Little Rock he could not have held it.
In actuality a more realistic target than Little Rock for Curtis' advance would have been Pine Bluff, as at Helena he was effectively boxed in and made no further significant contribution with his army for another year.