Why did the picnic sign change?

In 1991, the Office of Safety and Traffic Operations of the Federal Highway Administration sponsored a project to evaluate "the effectiveness of symbol traffic signs for young, middle-aged and elderly drivers." This document, Symbol Signing Design for Older Drivers (FHWA-RD-94-069 in PDF) was published in July 1997.

One of findings (pg 63) was that the small detail of the "picnic table beside a leaning tree" made the sign slightly more difficult to understand. The report concluded that distance would only increase that difficulty.

So, while the 1988 Manual on Uniform Traffic Control Devices (MUTCD 1988, pg 2D-26 in PDF) does indeed include the "picnic table beside a leaning tree", the MUTCD 2000 (PDF) does not. The 1997 report seems to have signaled the end for this icon.