It's particularly odd because he was writing for DirectX/x86, and thus had lots of power compared to the contemporary console hardware. Wolf3D(1992) used C; Ultima 7(also 1992) used C++. RCT came out in 1999, when this transition was essentially complete, with the possible exception of a few holdout platforms, e.g, Game Boy Color.
On the other hand, Sawyer did have some rationale to continue going all-ASM, since he had been building up the engine code over the course of the decade, and when he first made it for Transport Tycoon(1994), hand-optimizing the rendering code was probably the only way to achieve a smooth high-res isometric renderer.
