Last time I spoke with Scala IDE developers in person, I've asked a similar question and the reply was "SBT support was abandoned, because it is impossible to implement".
I just hope they were overly dramatic ;) Seriously, though there are very specific technical challenges: SBT project model (eg. .sbt files in root + .scala files in /project, and project/project nesting) are hard to represent in Eclipse Workbench / JDT model.
Without that you can't really have an .sbt file editor with correct classpath management which is in turn needed for auto completion suggestions and errors as you type.
Integrating sbteclipse plugin into the IDE itself is a much more tractable problem, but there is number of corner cases that result in generating incorrect Eclipse project definitions that need to be tweaked by hand. Which, as I understand is the reason the developers were hesitant to work on in.
Cheers,
Rafał