We built a bespoke CMS which we used for all our clients, but versioning was hell. Any time we'd fix a bug on our latest version we'd have to revisit all the clients on lower versions and manually add the fixes. Around 2007 / 2008 we realised that using WP as a framework meant we could extend it with whatever we needed and the update of core would take care of a lot of our donkey work. Add to that there were lots of handy functions and docs for the tasks we'd often perform, sanitisation, user-meta, uploads, etc.