Because they didn't know it was even there. They only encountered the 30 foot high outcropping when the Marines actually got in front of it. It did not appear on any maps, and proved to be an extremely formidable obstacle, even for tanks:

Worse, the face of this ridge...was honeycombed with caves and firing positions which the Japanese had blasted into the coral and turned into into excellent defensive positions which resisted all initial assaults.

Because 3/1 was directly in front of it, they had units on either side. In order for the Navy to blast it these Marines would have had to pack up and leave the beach entirely, which being DDay would have been a logistical nightmare, plus this would have made a huge gap in the line when holding a solid line was most critical. Secondly, even if the Navy had known about it, they had been shelling the beaches for several hours. At that time, they had no idea how many Japanese were actually there, nor how entrenched they really were. It is also likely (as it very well proved over the next month), that naval gun fire would only do so much to the fortified positions, especially blasted out of coral rock, and not sand or dirt. So by the time the Marines figured out what it was, they proceeded to attack as normal. Once this proved untenable, they had to try and flank it. Trying to back up and let the Navy blast it would only have meant they had to do exactly what they had been doing to take over the area and clear out the fortified positions not touched by the naval shells, which would probably be most of them.

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Quote from Battles of WWII: Peleliu 1944 by Jim Moran and Gordon L. Rottman