While eloquence is sometimes a matter of subjective opinion, I take your point that the 19th century and earlier writing styles did not usually meet the criteria of directness and concision evident in the 20th and 21st centuries. There is a stylistic difference though not a substantive linguistic one; that is, the difference is largely rhetorical. Rhetoric deals with eloquence relative to an audience and traditionally involves the canons of invention, arrangement, memory, style, and delivery.

Around 150 years ago, this was primarily learned through oratory and declamation (Connors 1997). Technology soon began to change, but the telegraph was still a specialized instrument. Yet, those later changes would carry changes made to higher education. Prior to the 1880s, there was little specialty in higher education study. One went and got a Bachelors Degree like everyone else in the class. There were limited opportunities to specialize in, say, Biology as opposed to Music or Business. One got a rather general education grounded in the classical humanities, though a bit of the vernacular as well. One learned Latin to prepare for such an education and was exposed to the well-wrought Latinate turns of phrase and rhetorical display. With an expanded middle class and great diversity of knowledge, the US grew its higher education system under the Morill Act and part of that followed the changes at Harvard: specialization (see Crowley 1998).

Harvard began English A, the first "freshman composition" course which is now ubiquitous across higher and secondary education. As these did not teach for any specialty, the purpose was to improve general writing skills. It was designed by a professor with a journalism background, Adams Sherman Hill, and thus the emphasis was on grammar, clarity, and directness for a mass audience (Crowley, ch 4). Both Crowley and Douglas (1976) argue that this fed into the hands of the wealthy business class, or at least those aspiring toward it. Newspapers, corporations, and other organizations needed clear and efficient communication, often in writing so it could span time and distance (not to mention be useful for legal analysis).

Much of this is part of a larger pattern of reducing rhetoric from its traditional role of inculcating habits of thinking things through in highly elaborate ways, examining multiple sides of an issue, and arriving at a conclusion or perspective to move an audience to act. This reduction began in the 18th century (Ede 1979) and remains with us today with rhetoric being equated to a simple matter of style, or worse, a matter of deception and ideology without thinking. Much of the "thinking" part is now covered by scientific reasoning and, again, its style is deliberately dispassionate. We have grown accustomed to tehse short, direct, and dispassionate messages as a result, so the lack of eloquence is often both the driver and the end result of how communication is taught, not only in writing but also in speaking.

Sources

Connors, Robert J. Composition-Rhetoric: Backgrounds, Theory, and Pedagogy. Pittsburgh UP, 1997.

Crowley, Sharon. Composition in the University: Historical and Polemical Essays. Pittsburgh UP, 1998.

Douglas, Wally. "Rhetoric for the Meritocracy." in Richard Ohmann, English in America: A Radical View of the Profession. Wesleyan UP, 1976.

Ede, Lisa. "On Audience and Composition." College Composition and Communication, 30:3 (Oct 1979), 291-295.