0) JPEG is past, present, and near future. Well supported everywhere.
1) JPEG optimization could be better. Mozilla is doing more of that.
2) Patents on enhancements to JPEG from minor obvious ones to significant compatibility breakers prohibit improvements. JPEG's final compression step was poor from the beginning and the better stuff was patented and unused. At least a decade ago StuffIt used modern binary compression to replace the final phase, which was exempt from existing patents; however, StuffIt patented this idea... it increased compression by 20-30% with no loss.
3) JPEG 2000 used a more modern encoding process probably similar to VP8 as far as the quantization and color space handling. Nobody uses JPEG2000 even though the smarter encoding makes compression artifacts less noticeable. The file size was not much smaller for all that extra RAM and CPU it took over JPEG - plus incompatibility. probably patents involved with it's demise as well.
4) Yet another JPEG replacement by a big corp... When everybody has MORE bandwidth, faster computers, and unimaginably more storage space on their smart phones than their desktops had in the late 90s. Why limit yourself with an obscure format to gain 30% more space when the storage you buy on sale at the end of this year will easily be 30% bigger than last year.
How much more CPU / watts will WebP cost you over JPEG?
I don't care how good it is; it has to be patent free, widespread, and proven.