For the exchange to handle deposits and withdrawals effectively, the platform must run an instance of the coin's daemon continuously and have it connected to the main network of the altcoin.
Wallet maintenance can mean several things, including the following:
The exchange's wallet is out of sync with the correct fork and needs to be resynchronized.
The exchange is currently implementing a wallet update released by the developer.
The wallet daemon on the exchange's server has encountered an error and needs to be restarted.
To avoid any requests that cannot be fulfilled, the exchange may disable the asset, since the corresponding wallet doesn't work properly. In this case, the exchange informs customers that the asset is under maintenance, which means that the exchange is unable to provide its customers with access to the actively running and synced client of the altcoin.