I have to confess that, after the initial excitement around Incanter, I haven't kept tabs on it.
Last commit to develop branch: Dec 11, 2016
As for why Neanderthal doesn't support core.matrix, I don't know. I see the author is active in this thread, so he can probably give a much better answer, but from reading the mailing lists I gather he mainly created Neanderthal to scratch his own itch, so core.matrix support wasn't a priority.
A community member of ours made an implementation of core.matrix backed by nd4j a while back: https://github.com/ds923y/nd4clj
It needs a bit of updating though.
We support cpu and gpu cublas and arbitrary matrix operations. We have our own memory management engine among other things built in.
This lib will also support automatic differentiation and sparse in the next few months.
I asked why neanderthal wasn't part of core.matrix and Mike replied that he'd like if it were 'for the sake of the ecosystem', and he'd offered some code and tests last year to get this started but it didn't get taken in. It's still there from last October, probably a bit out of date by now...