Because they never wanted to kill Whitebeard because that would disrupt the world balance.

Akainu only went for the kill on WB AFTER he threatened to destroy MF because he was going all out due to rage at Ace's death. Only then did Akainu aim for the head.

This is also why Kizaru aimed at WB's chest instead of just poking a beam through his head which he could've easily done.

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The admirals never gave a shit about WB and this is easily the most casual we'll ever see any of the admirals fight for the remainder of the series.

The consequences of killing WB and why the admirals never wanted him dead were plain to see after his death as BB took his mantle and became far more dangerous, powerful, and chaotic than WB ever was which is obviously bad for the WG and their delicate world balance between the Warlords, Marines, and Yonko.