Probably because those distros build their own packages for popular software, because they have more people working on it. If we had more manpower, then we would probably do so as well. For the time being there's only two of us developing Sparky. Pavroo is busy making the current Sparky-specific packages better and preparing for the big repository shift once Debian releases it's next stable and I'm still on making the ISOs boot on EFI32 and implementing it into sparky-backup-sys (but given the amount of time I have lately, it's still quite a way from being ready). Then there's the forum, where there's a few more people helping us out, but with bigger isssues we have to dedicate what time we have to help our users ourselves, so we're quite engaged over here.
Things will start speeding up once I get the build system up and running (package building automation with minimal effort), but I only made a concept of it and no physical implementation yet. I also had to give up on Calamares for the time being, so the live-installer will stay a while longer.