Currencies don't always need a "symbol" for a particular denomination.  Frequently a nickname is sufficient.

As examples with U.S. currency, please explain what the "symbol" is for the following denominations:

1 Lincoln (or fin)
1 Sawbuck (or Hamilton)
1 C-note (or Benjamin)
1 large (or grand, or G, or K, or stack)

"bits" is simply a popular slang nickname for micro-bitcoin in the same way that "grand" is simply a popular slang nickname for thousand dollars.

People are happy to use an extra character to indicate a larger scale of money ($5k) instead of ($5,000). I don't see any reason why it should be any different for the smaller scale of money (BTC5µ or µBTC5 or 5µBTC) instead of (BTC0.000005 or 0.000005BTC).