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Can you explain why a wallet should only be used once and not re-used?

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There are at least three theories I've come across to not re-use [addresses].

1. Once a key is used, it is technically part of the block chain, and while it may be encrypted, the fact is hackers could work enormous calculations to crack it. The reason your bank login is safer, is your bank will likely allow three attempts and lock it down. By comparison, anyone can download the blockchain and run infinite cracks to dissever the private key data, possibly enabling theft. Keep in mind, hackers do not need your address, only the private key which contains your public address within it.

2. Once you transmit your key over the internet to "use" it legitimately in the blockchain to send Bitcoin, although encrypted, it is subject to capture and hacking.

3. With common virus distributed key stroke loggers (etc.) your copy and paste of the private key to use it, could be detected compromising your wallet security.

Again, all theories, but we have had theft in at least 1 of these categories and the list is not complete by any means. Do a test, leave a sloppy wallet out there with a few dollars in it, if it gets hacked, you know there is a security hole in your strategy. I would also consider BIP38 encryption for your private keys if you discover any breeches in your system.