Aside from specifics about these people, the reason for Ancestry/Familysearch is due to multiple reasons:
1. Free. Free, free free, free free free. Familysearch is completely free, but Ancestry isn't always. However, many folks' public libraries (like mine) allow access via the library account, so it's still effectively free.
2. Access. So many people have Ancestry accounts or know someone who does, that it's one of the major players for genealogy in the western world. FamilySearch has been the go-to place for records for....ever? The LDS organized and created the GEDCOM format, drove several of the software options to create family trees, etc. It's important to them, and we benefit from their investment.
3. DNA. Ancestry allows users to test their DNA, and is an easy way to connect to others who have done this. This is also one of the major focuses on WikiTree, so they complement each other.
4. Digitization. Both FamilySearch and Ancestry are official partners of the U.S. Government to assist in digitizing census and other records for public availability. More and more are becoming available, and their work allows us to not only find things, but search within those collections in ways that did not used to be possible.
5. Depth of information. Both Ancestry and FamilySearch are much more than BMD and census records, they have multiple collections that include things like draft records, pensions, places where people can just post family memories or family trees, etc. While they may need to be taken with a grain of salt and sourced properly, they can be very useful. Both organizations also have invested time and energy into training materials, cooperation between sites, conferences, etc.
So, while there are still better places to go if you are not looking for US/Canadian records (and limited UK/European records), these two overlap quite a bit with the WikiTreers' interests, and continue to expand to a more global reach as time passes.