The short answer is, you need to study the circuit layout of a suitable device to see the design, and from this you can possibly work out why.
My gut feeling is that the design calls for interfacing with 5v TTL, but the device itself wont function at this voltage, exactly how it does function requires a suitable example to study.
This is easier said than done, as I can find very few details on the web.
What I did find was a wealth of info about the 8008, which predates the 8080 by a couple of years, this information includes ... a partial schematic, which you can find here.
Take a look round about page 29 and 30 (these are the page numbers of the pdf, not the hand scanned manual) and even page 5 if you want to see how it is physically constructed.
You can find more info here.
I don't expect any bounty for this, as I haven't directly answered the question, but I hope it points you down the correct path.