Also the point that now Powersaves (or whatever the new version of AR is called) is a thing again, so people can just spawn in as many Bells as they want. You can't exactly "spawn in" TBT, so TBT gets a lot of worth because of that.
I mean granted, the "hacking" is nowhere near as bad as it was in Wild World, but it still happens.
I think a lot of the reason why TBT is used over IGB is convenience. If 100 TBT equals 20 million IGB, it's a whole lot easier to transfer the TBT than it is to do however many trips it takes to exchange that much IGB.
I personally value IGB over TBT because I still need bells to pay off home renovations and TBT is easier to earn for me as I can just post to earn them. Collectibles here don't matter much for me.
That said, I can see why people like selling in TBT. Much more convenient and it can be used to buy collectibles. If you've already finished your home renovations and bells are no use for you if you dupe, TBT is probably worth more for you.
That being said, ever since the update TBT has been worth less. I've seen 3Ds selling for 200 TBT...like really? For a cheap item almost anyone can get? It's ridiculous how much things cost now.
It's only worth less right now because there's a huge influx of old members who have hoarded tons of TBT over the years that haven't actively been playing ACNL. They can spare the amount to pay for things. Honestly, once you pay off your house loans you dont really need a lot of bells :s