Having the communication in cleartext also makes it much easier for attackers to interfere with!

Sure, they can't modify the .deb without failing signature verification, but they _can_ inject arbitrary delays in downloads or interfere with anything else which isn't signed (e.g. HTTP headers)

Plus, if a vulnerability was discovered in the signing tool which enabled signature verification bypass with a certain signature format, HTTP makes it easy for attackers to perform that attack.

TLS shouldn't be optional for installing packages today IMO - the extra guarantees it provides are worth it even with signature verification enabled.