Two main reasons that I can gather from the comments is that replicas are so popular because the buyers can't fit into regular brands sizing or they cannot afford brand dresses.
In which case, then I ask rhetorically why is it that the most popular replicas are usually of prints like Moi-meme-Moitie's Iron Gate, Holy Queen. Angelic Pretty's Sugary Carnival, Star Night Theatre, Milky-chan etc., ahem. So it's actually the prints that are really being desired here? Ah well....
Being a bigger size loli, I can understand the frustration of not being able to fit into a size 2 Juliette et Justine dress or even an Innocent World's 98cm bust jsk. It can be quite demoralizing. Whether it's just a fashion or not. It's an unpleasant reminder that much of general society, even a fashion subculture, views you as physically unappealing. Although I know I'm luckier as there are still brand dresses that I can fit into as long as the shirring is forgiving.
If one is unhappy about sizing problems and just want a loli dress that fits well, be encouraged! There are a growing number of indies loli brands out there to explore and which can custom-make dresses as well. And some of these brands are even branching out into creating original prints of their own, after starting out by doing replica prints, which is nice.
Taobao can be a great place for custom-make loli that is affordable. It does take some navigating and a reliable buying agent but the returns can be quite worth it. I use a local agent, Peeka, and so far, I have no complaints. They've been good, speedy and their system of ordering from Taobao is wonderfully easy! I'm not sure if they can do international orders but it doesn't hurt to ask =)
Kidsyoyo has an original print, Alice Falling Down, that is gorgeous! Such a pity it's already sold out. I would have liked to order one.
While Krad Lanrete has a lovely OP that's fantastic for a mature lolita too.
And I love the red version of this!
Akane&Alois is another Taobao-based brand to notice. Their stock is relatively limited but what they have is mostly classic and very pretty.
Akane&Alois's Leaves of Grass jsk
I've just ordered their new Anne of Green Gables outfit. It's not really loli unless one stretches it by calling it country mori/loli I suppose....anyway, I'm preparing it for my eventual visit to Prince Edward Island, where the Anne of Green Gables books are mostly set, haha. Will do a review post when I get the dress!
Other stores to consider for all your custom-size loli needs:
And do be careful when shopping on Taobao though. Quality can vary and what you see on the site may not arrive exactly as pictured. A little research never hurts! If you see a dress that you like, check out any reviews available on it. Chances are someone probably purchased from the shop before so poke around the memories section at egl. And stay away from shops that re-uses brands' photos! They usually turn out to be crap, and I kid you not. Milanoo, milanoo!