Because projects which are using LGPL can be included in proprietary applications. Those application must not provide the source.

With GPL you can only include the application in other open source software.

This makes a big difference.

Perhaps the developer wants to avoid that the software is included in commercial software. This is one of the main reasons using GPL over LGPL.

Old version which are licensed under LGPL can be still used, but they wont receive updates.

Most developers are switching to a proprietary license to make money. If you have used LGPL code and you need an update to the latest version, you gave to buy a license.

So switching from LGPL to GPL is a good thing for an open source project.