The logic seems to have been:

* Ubuntu 18.04 is a LTS release.

* LTS releases should only incorporate software that is supported on a similar lifecycle.

* OpenJDK 8 was moving out of its support window early in the lifetime of Ubuntu 18.04.

* OpenJDK 11 (the next long term supported JDK) was not going to be released until after the release of Ubuntu 18.04

* OpenJDK 9 and 10 were short term releases so they weren't options

* The move from Open JDK 8 to Open JDK 9 included a lot of breaking changes (from project Jigsaw/modularisation)

So a compromise was necessary. From a purist packaging point of view it makes sense; release JDK 10 as "JDK 11" and silently upgrade under the covers when possible. In theory this would only involve minor changes and the 'wrong' version would only be there for the first few weeks of the release. In practice there were some breaking changes - enough that the backporting of OpenJDK 11 into Ubuntu 18.04 actually took rather a long time. It's also, of course, really confusing when you "sudo apt install openjdk-11" and something totally different happens.

The problem with staying on OpenJDK 8 (perhaps the obvious approach) is that in an unsupported state it would lack security fixes. In practice of course several alternative upstreams are now available supporting it.

Hopefully it will all go with fewer surprises when we get to the next LTS edition (Ubuntu 20.4).