why Blitzmax has a reputation for game design
If by 'reputation' you mean people consider it is for games then that could be due to what people know about it. It has got Blitz in the title. Blitz is often related to BlitzBasic (B2D/B3D). Lots of games have been created using Blitz software. Those that care often pay more attention to the language a game is written in than an application.