why SAP programming has (almost) no representation on Stack Overflow?
I've often wondered this myself. SAP seems like the perfect subject for a Stack Exchange site — so I suspect there has either never been a push internally from that community to rally support here, or perhaps few people have recognized Stack Exchange as accepting such questions.
Keep in mind that the biggest challenge (and risk) facing a proposal is the lack of support from a well-established community already active in that subject space. Area 51 no longer supports a model where folks simply propose subjects without establishing support first, so be sure you organize enough active users ready and willing to support your site before submitting a proposal.
See: Minimum Number of People For a Successful Proposal
technical traffic toward Stack Overflow and functional topics toward a new Stack Exchange site
There are no features to facilitate this type of division between sites, nor is it particularly advisable. Each community has to stand on their own as a comprehensive subject — some overlap of topics is okay between sites — but we don't want to create a situation where folks looking for help find their question being tossed around like a game of hot potato because the nuances of what belongs where creates a heavily gerrymandered scope.
For subjects which typically include an element of programming, such proposals are generally allowed if they are able to demonstrate that there's also a sufficiently large scope of questions which cannot already be asked on existing Stack Exchange sites. That has to happen quickly (in Definition), so be sure to gather enough support from folks who are actively interested in building and using the site.