That's because he is slower in newer games. Every single 3D Mario since 64 toned down his speed, flexibility and inertia to appease a more casual style of Mario gameplay. 3D World is probably the fastest he's been since the '90s, but it's still not up to what he can do in 64.
Odyssey looks faster still.
This is a very serious post to analyze. I'd like to focus on the 3D Mario path that goes Super Mario 64 -> Super Mario Sunshine -> Super Mario Odyssey. Our variables are (A) speed, (B) flexibility, and (C) inertia.
(A): Mario has greater speed from a standstill in Sunshine than he does in 64. This is evident by the fact that Mario can dive forward without any prior momentum. When it comes to flat surface speed the fastest move Mario has ever had in 3D is FLUDD's waterslide move. This can even be done from a near standstill position if the player activates the hover nozzle and then immediately dives. Now when it comes to max speed levels Mario can attain Super Mario 64 is the clear winner with diving wingcap maneuvers. I'm also fairly certain that using sharply inclined planes puts Mario at his second fastest speed in 3D history. All of this is ignoring the advanced technique of backwards long jumping of course.
(B): Flexibility overall is generally the same, but each Mario can be seen as very distinct from the other. Mario in 64 has special access to crouch movement: crouching, crouch walking, breakdancing (well, low kicking), slide kicking; combat: punch, punch x2, punch x2 + kick, and jump kick; and handstands (i.e. at the top of trees).
Mario in Sunshine has spin jumping, grate grabbing, wall sliding, two types of flips out of dive, hovering ("flexible" aerial mobility), full body turning through Y-Cam, and some other stuff I can't remember right now. This is ignoring advanced techniques such as changing linear aerial movement to analog aerial control while retaining fast forward movement by activating Y-cam and Turbo Nozzle while airborne (this cannot be done if you started from a grounded turbo boost).
Each game retains the other elements. [Sidenote: The most noticeable change in Galaxy was the dramatic reduction done to Mario's walk/run animations.]
(C): No contest here. Sunshine nerfed inertia hard.