Because China wants to dominate this region and recently signed a secret military pact with the Solomon Islands, for example:

China’s influence increased significantly during the term of Manele’s predecessor, with the country switching diplomatic recognition from Taiwan to China and striking a secretive security pact that has raised fears of China’s security forces gaining a foothold in the region long dominated by the U.S., Australia and New Zealand.

…

The security pact has raised concerns of a Chinese naval presence in the region and the basing of ground troops that could challenge other countries’ access to the region that lies close to Australia and over which Japan and the U.S. fought savage battles during World War II.

https://apnews.com/article/china-solomon-islands-us-taiwan-pacific-dc2e984ec5045060524c644ebf62b910