Probably because this version is not released in the repository. This is something we need to adjust maybe in the AS 7 plugin or in the nexus repository itself.
For the meanwhile, pick the 7.1.1 version.
As George said it's because there is no full 7.2.0.Final distribution, only components are available. The last full distribution is 7.1.1.Final, at least that's available to this plugin.
I will note that this really isn't an issue with Forge as much as my poorly written plugin that badly needs some love :-) Hopefully very soon I'll be able to update this plugin, fix the poor error messages and show versions that are available for download.
You could manually install EAP 6.1.0.Alpha and point the plugin to use that. I realize at this point the prompts can be confusing, so feel free to ask any follow up questions if that's the route you decide to go.
James R. Perkins