That seems odd to me - why could one not install (or build) package N-1 in order to build package N ? i.e. why does it all need to be the same package name?
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It would have required moving openjdk-9 and openjdk-10 into backports only to be removed later. These packages were already removed from unstable and testing, and I assume it would have made things only more of a hassle... Also I guess it meant that installing openjdk-11 on stretch-backports would eventually gain you Java 11 (and it does now).
I'm not a Debian maintainer. They'll probably have a better answer if you asked them :)