Outside of late Beethoven (which a lot of commentators of the time considered incoherent), I don't know any other music in the 19th century that preceded Wagner and had anywhere near the rate of modulation of, say, Tristan or the Ring cycle. That alone must have made it seem to listeners that the music is simply faster or somehow more compressed with meaning than nearly anything in Opera that came before.
That's an enormous qualitative change that likely held sway over listeners as trance music does for newcomers to a cult. And like the introduction of any new technology (whether it's a compositional technique or equipment like a loudspeaker), any philosophy/staging/prejudice/long-windedness you pipe through it is going to get boosted by that novelty, well past the meaning those things would have carried without it.
Digression-- to me, it's a bit of a shame that both "camps" of the latter part of the 19th century-- Brahms and Wagner-- were essentially maximalists. Whether it's the 1st movement of Brahms' 4th symphony or Wagner's Tristan prelude, both composers are going to shuttle a tiny, self-contained motive through every harmonic, textural, and contrapuntal context they can write out before the music ends. This formal strategy persists into modern composition to this day, with a zillion pieces that start with some primordial "seed" from which the entire piece seems to generate its own life. (And the composer ensuring you hear the entire damn life cycle lest you doubt their abilities!)
On the other hand, there were composers like Debussy who seem to have worked out what their "seed" can grow into ahead of time (or else just intuitively grasped such a thing), and then choose for the composition only a few uses of a motive which were the most evocative. Apples to oranges of course, but... instead of 1000+ appearances of the Tristan chord, Debussy's "reflets dans l'eau" gives you four appearances of a little oddball motive-- each one slightly mutated (with the final one even seeming very much like an allusion to-- and rejection of-- something from Wagner's Ring cycle). What happened to get that little motive from point A to point B is left to fan fiction rather than explicitly worked out in the composition. Doing that coherently is a much more difficult task. While a lot of composers borrow the harmonic sounds of Debussy, I don't hear a lot who effectively emulate this part of his writing.
Edit: Hm... rate of modulation isn't really the right metric. It's more like average rate of modulation, along with rate of textural change.
For example, you quite a bit of harmonic turmoil and quick textural change in the development of a Chopin Sonata. But it's surrounded by sections that aren't as harmonically active.
Both Brahms and Wagner flip that pattern around so that the meat of the composition is in flux. Brahms' development sections actually provide respite from that; or, Wagner gives the listener a harmonic acid trip for nearly two hours until the lovers "extinguish the flame" and things start to get in a groove for about 15 minutes or so... :)
It's the same set of techniques. But the fact that Wagner insisted on sustaining that rapid developmental material for the greater bulk of an evening, or multiple evenings, was clearly revelatory for a lot of his admirers.