The question of why US copyright law is administered and, to some extent regulated within the Legislative rather than Executive branch has been raised in a dead thread. The cogent point is made that under the US Constitution, the phrase "checks and balances" generally applies to both the division of powers amongst the three branches (Judiciary in addition to the two previously mentioned), and the principle of review and oversight amongst those branches (e.g., legislation is passed by Congress, approved or vetoed and administered by the President / Executive branch, and subject to interpretation or invalidation by the Judiciary; executive appointments are subject to Congressional approval; and members of both the Executive and Judiciary may be impeached and removed by Congress).

That said ...

... there are other instances in which separation of powers is not strictly followed. Examples which come to mind are:

- Administrative law judges (ALJs), notably in matters concerning Social Security and Immigration law, being a judiciary function under the executive.

- The Sergeants at Arms of the US Senate and US House, both legislative bodies, but performing executive functions. Recent history suggests that the Executive cannot be entirely relied upon to provide this function.

- Judicial Review is probably the biggest appropriation of powers, in which the US Supreme Court arrogated the right to rule on, interpret, and invalidate legislation. This is a power arguably derived absent any constitutional, legislative, or executive foundation.

And of course the present Administration has increasingly expressed a philosophy not only of Unitary Executive, but increasingly of Unitary Government, enacting law by decree, executing citizens without due process, and openly flouting courts. H.R. 6028 could be seen as part of this expansion of the Executive.

Which still leaves us with the question of how Congress ended up administering copyright.

I don't have a full history, and have only been exploring the question for the past hour or so.

The US Copyright Office itself has a history page noting that:

On July 8, 1870, Congress centralized the administration of copyright law in the Library of Congress at the encouragement of Librarian of Congress Ainsworth Rand Spofford.

<https://www.copyright.gov/history/copyright-exhibit/history-...>

Which remedied the previous arrangement in which Copyright was administered by ... the Judiciary.

Why Congress ended up regulating copyright is probably largely a set of historical accidents and conveniences. The Library of Congress does in fact serve Congress (and IIUC the Judiciary, to which it is also proximate) as a legislative research tool. I've read enough of the annual reports in the latter half of the 19th century to know that the Library was growing rapidly at this time, and was constantly pressed (literally) for space, culminating in the commissioning, construction, and opening of the separate Library of Congress Jefferson Building, in which the main collection is now housed. (As I'd recently commented, there were concerns at the time of how merely moving to an adjacent building might affect retrieval time for materials.)

Arguably, the US Library of Congress had, and still has, more expertise in the management of large corpora of physical publications than virtually any other institution on Earth. Copyright registration itself served the interests of Congress by growing the collection. And as of the late 19th century, the overall size of the US government, though growing, was still comparatively small. The Executive would possibly have had neither the interest nor capacity to administer the Library, or even the Copyright office sufficiently, nor the convergence of goals in growing the Library's collection noted here. Given numerous issues with other areas of intellectual property which are administered under the executive (patents and trademark, though my criticisms are largely of the former), its also possible Things Could Have Gone Badly Wrong, though arguably as the EFF piece notes they have already. Though the House legislation seems likely to worsen that.

The present situation though is that the Library of Congress and Copyright Office do strongly blur the separation of powers principle, affording a complex set of legislative, executive, and even judiciary roles, all under the Legislative branch.

That just my own nonexpert nutshell summary. If anyone has further information on the history of the US Copyright Office, legislation, and judicial rulings, please pitch in.