Contiki 1.x started out as hobby project on the C64. It was ported to quite some similar machines by several developers. There's a Hitmen page dedicated to Contiki 1.x with multiple screenshots. The final source code was made available as GitHub Contiki 1.x repository.
Contiki 1.x featured a GUI with a drop-down menu, movable windows and icons. However, more complex programs like the Contiki web browser required so much RAM that no other program could be run beside them.
Contiki 2.x was targeting wireless sensor networks. That meant that both core parts of Contiki 1.x (the IPv4 TCP/IP stack and the GUI) weren't relevant anymore. Oliver Schmidt became the only developer maintaining Contiki 2.x for desktop machines. Given that there wasn't enough RAM to run more than one relevant program at the same time anyway he decided to drop the menu, windows and icons. Instead each Contiki program became an individual standalone program running the GUI in full screen mode. The RAM freed up this way allowed to activate Contiki features (like e.g. mouse support) previously deactivated because of RAM shortage.
Contiki 2.x started out with more desktop programs than delivered today. The primary reason is that some of them were designed to communicate with peers which became irrelevant, like SMTP servers without encryption or FTP servers without Passive Mode.
In the meantime Contiki development has moved on to the GitHub Contiki-NG repository. Given that neither the IPv4 TCP/IP stack nor the GUI aren't relevant anymore it's reasonable that they have been dropped from that repository together with all desktop programs.