Sun leadership wanted to license OpenSolaris under GPLv3. However, GPLv3 work was dragging on at FSF and the license was not released in time. Moreover, there was opposition from Solaris dev team due to belief that GPLv3 will lock out reuse of OpenSolaris code (especially DTrace and ZFS) in Free/Net/OpenBSD.
CDDL was a compromise choice that was seen as workable for inclusion based especially on certain older views on what code will be compatible or not, and it was unclear and possibly expected that Linux kernel will move to GPLv3 (when it finally releases) which was seen as compatible with CDDL by CDDL drafters.
Alas, Solaris source release could not wait unclear amount of time for GPLv3 to be finalized
So... as I said "Sun explicitly did not want". They chose not to license it under GPLv2 or dual license GPLv2 + GPLv3 for... reasons.
> it was unclear and possibly expected that Linux kernel will move to GPLv3
In what world? Kernel was always GPLv2 without the "or later" clause. Kernel had would tens of thousands of contributors. Linus made it quite obvious by that time kernel will not move to GPLv2 (even in 2006).
Even if I gave them benefit of the doubt, GPLv3 was released in 2007. They had years to make license change and didn't. They were sold to Oracle in 2010.
CDDL was a compromise choice that was seen as workable for inclusion based especially on certain older views on what code will be compatible or not, and it was unclear and possibly expected that Linux kernel will move to GPLv3 (when it finally releases) which was seen as compatible with CDDL by CDDL drafters.
Alas, Solaris source release could not wait unclear amount of time for GPLv3 to be finalized