There is one significant difference between preparing and maintaining releases for the Lite (IE only) version and single-engine versions that would use the Gecko, WebKit, or Chrome engines. The Lite version does not include a rendering engine in the download at all, since it uses whichever version of the Trident engine is already installed on users' systems. In the cases of the other three and of the Ultimate version, the rendering engine(s) must be included in the downloads. This means that, for the WebKit, Gecko, and Chrome versions, every time Avant is updated and every time one of the engines is updated, the corresponding single-engine version would have to be updated in addition to the Ultimate version. Add in the USB versions, and you have quite a bit of additional work required to keep everything current.
My point is that Anderson would have a lot more work to do to get each update ready for release, and he'd also have to update both the Ultimate and one of the single-engine versions every time one of those engines is updated. There would also be more work and time required to keep track of all the various releases to ensure that all of them are kept current. I'm in no way against the single-engine versions, and I agree they'd be good to have available, but I doubt that Anderson has the bandwidth to handle the extra workload, and since Avant is supported only by voluntary donations, I doubt that he can hire additional programmers to carry the extra workload.