The issue with usermade board-sets is that it potentially threatens the commercial property of the game. Unlike the original DX-Ball by Michael P. Welch, which was released as a freeware game, DX-Ball 2 is released as a shareware demo. In that sense, when you install the DX-Ball 2 demo on your system, you are technically installing a demo version of the commercial product Board Pack 1.
Although usermade board-sets was considered to be supported for users who had purchased Board Pack 1, things took a different turn after Seumas passed away, back in 2000. Notably, Longbow Digital Arts was an independent family business back then, Seumas being the president and programmer. Because of this threatening loss to the business, the company had to focus on sustaining the sales of additional board packs, which lead to the subject of a board-editor being discontinued.
True. A good idea that would both let LDA get profits from the official boardpacks and let users make their levels would be to include an official board editor with DXB2 when someone owns ALL the official boardpacks, and make it so that usermade sets can be run only when all official sets are installed.