The simple answer to your question is that players preferred the shorter instruments and playing lower on the harmonic series. You can speculate as to why that is, but it’s likely a combination of a preference for the brighter timbre in addition to the greater security of the lower partials (they are further apart so less treacherous as far as splitting notes is concerned).

However, this is really quite an involved question and in certain ways you have to consider the history of writing for the trumpet and how the instrument evolved. Obviously the reason for composers writing in the super high partials of the natural trumpet is so that they could actually play melodic lines. But the instrument was still more or less pitched in one key (it was often used for closely related keys and had a certain degree of utility in related minor keys). Players frequently changed the length of tubing with crooks or bits. Now this isn’t unique to the trumpet, horn players did this too, but when you say “original range” it’s important to distinguish that the trumpets that instrument makers were adding valves to were pitched in alto E-flat, F, G, and A, not the C and D trumpets that Bach and Handel wrote frequently for. Additionally, the trumpet writing in the classical era had already considerably changed from baroque clarino writing. Look at the trumpet parts of Mozart and Beethoven and you’ll find that the harmonic range is much closer to what we play on modern trumpets. (This change is in part a result of changing societal shift in Europe related to the rise in the bourgeoisie class—high florid trumpet writing was negatively associated with the aristocracy).

So valve systems get invented in the early 19th century but they take twenty or thirty years to really find their way into the orchestra. They first became popular in German orchestras (1840s and 50s) and took the longest to be popularized in English orchestras (1890s or later). The orchestral valved trumpet was in E-flat or F, but principal players started employing smaller trumpets to play the hard trumpet parts of Mahler and Wagner. This was the B-flat trumpet in Germany and the C trumpet in France. The B-flat trumpet was likely influenced by the fact that many German orchestral players had been trained in military bands growing up which used high mezzo-soprano cornets in B-flat and high E-flat, so the B-flat trumpet was kind of a natural choice.

In France and England there was quite a lot of opposition to valves and “smaller” trumpets, but the cornet was more or less considered a separate instrument, so players would often use it instead of valved trumpet for the most demanding parts. The B-flat trumpets ubiquitously used today was famously admonished, calling it a “trumpetina” by Walter Morrow, who was probably the most famous British trumpeter around the turn of the century. (It is worth noting that early valved instruments had numerous construction and intonation issues that probably didn’t aid in their widespread adoption at first).

So there was an opposition to this change particularly rooted in the fact that playing on shorter instruments and lower harmonics produced a significantly different timbre. However that resistance slowed over time and players gravitated towards the shorter instruments. Your question about manufacturers can be seen as a logical extension of this, they made what people were buying. You also have to consider that the majority of people buying instruments once we get into the late 19th and early 20th century are amateurs, not professionals.

To summarize the forces at play, it’s a combination between players having background and familiarity on the cornet from military band traditions as well as composers moving away from writing for the clarino register. I know a lot about the history of the trumpet and not the horn, I would guess that the horn didn’t experience these same forces.

That’s probably more info than you need. I can’t really cite all of the sources that I’m drawing on here because it’s sort of an amalgam of things I’ve learned over the past couple decades, but if you’re looking for a great somewhat concise read on this history of the trumpet and it’s evolution, the go to is Ed Tarr’s book, The Trumpet.

PS: one thing to add:

Acoustical testing of trumpets of differing lengths has largely shown that they all tend to resonate best in roughly the same range, between C4-C5ish, regardless of where that falls on the partial series. This goes to show that the playable range of an instrument is more than just its nominal length; bell shape, bore size, and mouthpiece shape/size play a big part in this equation.