It was never used in the park because it was deemed to sound too much like the beginning or ending of something rather than just part of an endless area loop.
I'm not sure where this information came from (I've seen it stated many times before) but this is untrue. Though "Welcome to Tomorrowland" doesn't appear in the Disneyland or Magic Kingdom TL loops, it does play in the Tokyo Disneyland TL loop. Each of the loops seems to contain at least one track not present in the others (Disneyland is the only loop to include The Monorail Song, and the Tokyo loop contains a version of the Timekeeper theme).
Randy is sometimes given material without note to where it is or isn't used, and there are several cases where cues not used in the park have been included on the albums. My guess would be that Randy was given all of the music recorded for the various TL loops and chose to include this particular cue, either not knowing or not caring that it isn't included in the US versions of the loops.