Why not Linux?

Linux-based distributions stack up software — the Linux kernel, the X Window System, and various DEs with disparate toolkits such as GTK+ and Qt — that do not necessarily share the same guidelines and/or goals. This lack of consistency and overall vision manifests itself in increased complexity, insufficient integration, and inefficient solutions, making the use of your computer more complicated than it should actually be. [top]

Instead, Haiku has a single focus on personal computing and is driven by a unified vision for the whole OS. That, we believe, enables Haiku to provide a leaner, cleaner and more efficient system capable of providing a better user experience that is simple and uniform throughout

via General FAQ | Haiku Project.

What platform(s) is Haiku targeted to run on?
The main target for Haiku R1 is the x86 (Intel, AMD, and compatible) platform. There are ports to other platforms underway, such as PowerPC, MIPS and ARM. However, it is not clear whether these will be supported or not. What platforms we support in the future will heavily depend on the availability of resources to support their development