It is my understanding that it was pressure from the Navy for the islands to be secured as quickly as possible. The admirals didn't want their ships stationary around an contested island for very long.
I have also read that there was friction between the Navy and the Army because the Army tended to attack in a more methodical manner. The more I watch this series, the more I am in awe of the Marines that served before me.
As I understand it, the Marine general at the time was part of the problem as well. I think Louis is correct in his assessment on this one:
"And so finally, it is the conduct of the marines themselves, specifically their leadership, that one must take to task. MG Rupertus was undoubtedly a tough, brave man. He had served ably in Guadalcanal and had been decorated for his leadership there. He had the utmost confidence in his men, their equipment, and his plan. American amphibious doctrine had been working across the Pacific with stunning results over the previous year, and Peleliu would be yet another validation of these methods. In these lies the greatest failing of Rupertus, and the greatest mistake by the marines: overconfidence.
Rupertus continually stated throughout the planning stages that the fight would be intense, but short. It would be a quintessential storm landing, with hard fighting on the beaches and immediately inland, but of short duration once the Japanese lines were pierced. He spoke of 'open season' on the Japanese once they began their inevitable banzai charges, and demanded that "somebody bring me the Jap commander's dress sword."(22) This was arrogance and overconfidence of the highest order, considering stiff enemy resistance continuing on Guam that summer, and the haphazard manner in which 1st MarDiv had trained and loaded for Peleliu. Morale on the part of troops and a commander's confidence are critical to victory, but such specific, bold claims on the part of a commander are irresponsible. Throughout the battle Rupertus, seemingly oblivious to the casualties his division was taking, insisted that the end was in sight, and that outside help was unnecessary. It took an order from his corps commander to get him to remove Puller's 1st Marines from the line, to be replaced by an army unit; Rupertus had repeatedly expressed his lack of confidence in the untried 81st ID, its commander, and the army in general. Apparently, it was a far better decision to sacrifice his own troops rather than take a chance on the army; inter-service rivalry is fine for enlisted men, but at the general officer level it is childish and counterproductive.
In addition to his unwillingness to accept help, Rupertus had planned an invasion for which he held only two battalions as his divisional reserve, with nothing else to back him up should the need arise. Apparently it was inconceivable to him that a situation in which his marines would need help could develop. Consequently, the 81st ID, whose attacks on Anguar and Ulithi were held up pending favorable progress on Peleliu, were launched. This, despite the fact that by D+2, the day Rupertus informed III Amphibious Corps that the 81st was not needed, 1st MarDiv was completely bogged down and suffering horrendous casualties. American intelligence knew that the garrisons on Anguar and Ulithi were smaller than the one on Peleliu, and this was precisely why the attacks on these islands were held off: so that the 81st could provide 1st MarDiv with support if needed first on Peleliu. Rupertus, stubborn to the end, had to be ordered to accept the army's assistance, and even then continued to insist that his marines would take the island shortly. Intermittent problems with water and food supplies,(23) along with stiffening Japanese resistance, and the high temperatures coupled with Rupertus' arrogant stubbornness probably resulted in more deaths than were necessary."
From:
http://www.militaryhistoryonline.com/wwii/peleliu/provencourage.aspx