MeeGo was mostly Moblin, even though the N9's MeeGo was mostly maemo. Moblin was mostly Fedora, although it switched to it from being mostly Ubuntu. Tizen is mostly .deb based SLP, although the Tizen IVI releases look to be .rpm based and maybe so have MeeGo (the Intel variant) background. Isn't it fun when you have all the hundreds of people working and you need something for them to do?
Of course the actual upstream software is mostly the same, but maybe it just makes you feel like running a big software operation when you're doing different packaging and patching and QA for all the packages every once in a while, maybe two packagings, tools and QA processes at the same time. With the exception that Tizen/SLP seems to use partially older software versions currently than its predecessors, like xserver 1.9.
I understand that if for example MeeGo IVI had already traction in the slow moving IVI scene, they don't want to disrupt that. I don't know, but nevertheless all these changes have been not only a pain for the community, but surely a pain inside the companies. Not that merging Debian and Fedora would be any more easy, but I guess no-one suggests that (except for the Canterbury Project - and not even that since it was only Debian, Gentoo, SUSE, Arch :).
Still, I'd value not spending huge resources re-packaging and re-QA:ing and re-process-evaluating everything all the time, because it causes friction in doing something actually new. Maybe Tizen however handles the current situation gracefully. If the base distro just happens to work (no matter how complicated its birth), the "new stuff" developers can concentrate on their work.