Because this has totally never been suggested. It can't be done in normal Blockland mapping programs because there are no released .csx versions of Bedroom and Kitchen, meaning we can't simply smack them together and add stairs. If someone is willing to take their time and make the map using the Blender files recently released by Soba, that would be fine, but it would be very complex and plainly frustrating.
The problem is when exporting to .map, the .map exporter cannot handle all those brushes, how did badspot get around this? he created the map in hammer and exported as .map from THERE
You can't just simply combine the default kitchen and the bedroom map together.
Badspot would have to release his source files to do so which I don't think he will do.
I sent badspot a PM couple weeks ago, he told me he used hammer, and the map2dif converter, In this PM, I added that I wanted to add onto the bedroom map, he had no problem with this, so I think its more of a "he doesen't have it anymore" thing