You could speculate that the 1900 was an expansion on the 200 which allowed Beechcraft (at the time) to add on to the existing Type Certificate Data Sheet (A24CE if you would like to look it up), and kept it in the FAR Part 23 certification requirements. Albeit in the "Commuter" category. Presumably much less costly to do than apply for a new TC in FAR 25. The 1900 is an "airliner" without meeting all the requirements for FAR 25, Transport category, over 5700kg.