Or maybe they removed the overworld Pokemon so they could put them into the third version and use that as a way to draw more customers.
...This.
They've consistently done this since 2nd gen.
Crystal adds animations!
Ruby/Sapphire remove them.
Emerald brings back animations, and adds a Battle Frontier!
Diamond/Pearl removes the Battle Frontier.
Platinum brings back the Battle Frontier!
It's a rule. The third game introduces something, then they remove it for the original games of the following generation and bring them back in the third game of the following generation.
Of course, HG/SS aren't third versions, but I'm sure it's just a slight variation on the typical formula. They just add something that people like, then remove it for long enough for you to get angry and then forget about being angry, and finally bring it back making you excited even though you should just be pissed that they took it out in the first place.
It's also worth mentioning that every time they bring something back, it tends to be worse than it was when it was introduced...the animations in Crystal were complex, now the animations are two frames with some shaking or something. The Battle Frontier in Emerald had 7 facilities, they brought it back in Platinum with 5.
So, I'm predicting that from now on the Battle Frontier will be a permanent fixture of the Pokémon series, as it's been introduced, gotten rid of, and brought back.