I saw some similar case with EOS as it had its own memo too, and that's because EOS was deployed on their mainnet, away from the ERC-20 (ETH token). I don't own IOST, but that could be the main reason.
How about downloading an IOST wallet, transferring them from MEW to it, followed by withdrawing to Binance?
EDIT: Binance just recently announced their support for the mainnet token swap
Binance would like to confirm support for the IOST mainnet token swap and will suspend withdrawals for IOST starting from 2019/03/08 0:00 AM (UTC). Binance will handle all technical requirements involved for all users holding IOST in their Binance accounts.
Please note that even after the IOST mainnet is online, if you choose to deposit ERC20 IOST tokens into your Binance account, we will continue to process the token swap for you.
We will make a separate announcement after the mainnet swap is complete to notify users when withdrawals for IOST (native coin) will open.
For further information, please refer to the official announcement here:
The Most Complete Mainnet Launch To Date: IOST - Olympus v1.0 Has Arrived!
To learn more about IOST and their new mainnet, read our in-depth Binance Research report here.
Thanks for your support!
Binance Team
2019/03/07
https://support.binance.com/hc/en-us/articles/360024642452-Binance-Will-Support-The-IOST-Mainnet-Token-Swap