Since TLS is preferred over SSL, why do we still use the terms SSL and HTTPS generally?

The former could be anecdotal, but most people I speak to still say SSL in general conversation. The term HTTPS is more objective, since that means HTTP over SSL.

Why don't we say HTTPT (HTTP over TLS) and use the scheme httpt://?

Huge effort. Little technical return.
Introducing a new scheme (schemes are e.g. http://, https://, ftp://, etc.) and deploying it would mean breaking backwards compatibility. Not worth it.
Political rather than technical
Ivan Ristic devotes some sentences in the introduction to his book to this.
The book is called Bulletproof SSL and TLS. You've got both the "SSL" and "TLS" right in the title. (Go figure.)
The introductory chapter is free online. The naming controversy is mentioned in section "SSL versus TLS" (page xix) and section "Protocol History" (page 3).
It seems the whole reason for renaming from SSL to TLS was political rather than technical. Ristic's footnotes link to the blog of Tim Dierks. Dierks wrote the SSL 3.0 reference implementation in 1996 and this is his take on the naming:

Tim Dierks, 2014-05-23, Security Standards and Name Changes in the Browser Wars (archived here):

As a part of the horsetrading, we had to make some changes to SSL 3.0 (so it wouldn't look [like] the IETF was just rubberstamping Netscape's protocol), and we had to rename the protocol (for the same reason). And thus was born TLS 1.0 (which was really SSL 3.1). And of course, now, in retrospect, the whole thing looks silly.



Further reading

Here's another take on the naming. It's by Mike McCana (who operates a CA himself):
Mike McCana, CertSimple.com blog, 2016-01-05, Why do we still say SSL? (Archived here.)

