Even if MTS2 does have a ".package only" upload policy that might turn people off from using it if they are used to Workshop, I bet the real reason is TSR offers rewards uploaders with something, even if they are just kudos (and as a FA she gets more that just kudos).
Also MTS2 does have "foreigners", like Masaharu777 that is Japanese.
Trouble is, many foreigners choose to use sims3pack file because it's practically easier to use. It also gives custom content the proper cc marker unlike .package which in some cases for some needed a special skill to properly place them in its folders.
Anyway, kudos is actually useless. It can only be use to personalize their profile page and to buy some kudos rewards. I guess MTS do offers page profile that costs you nothing.
I'm pretty sure the real reason is not about the foreigners thingy, she probably does want to share her work freely but the way she presenting it just isn't right. Well, she's a foreigner after all.