As i see from the updated description and collected comments for the question ("Why does Ethernet on UTP have much greater range than other modern protocols?"), the OP is interested in two aspects (and i assume, more much in the second :-) here:
1) common sense, that he defined as
I'm asking in electronics rather than networking since this is a physical link issue, not a networking issue.
2) particular sense, about that he let out in his comment
... it'd be nice to be able to plug in remote webcams (USB) a few tens of meters away for use as cheap security cameras, and to move a noisy computer to a closet 20m away from a display (Displayport).
Please me to start from the second aspect because it seems to me more simple.
20+ m USB link
There is no problem with such a length USB link. In that case, you need to use so called "active extension cable" (one or several stacked in series), like this, for example. It single can cover a span up to 20 m. Yes, in that case you need to power the distant cam separately (i.e. not by the AEC), but i think it's a small additional cost for playing that scenario. And yes, it's expensive (~100 USD/pcs), but the target cams are cheap :-)
For DisplayPort, AECs also exist, try this, for example. They are also expensive (~100 USD/pcs for the 15 m long AEC referenced). But as i understand you need only 1-2 pcs and only once for the next 5-10 noise-free years :-)
Comparing to them, a 20 m Cat5e UTP looks so cheap (~10 USD/pcs) and then magnetically, but an Ethernet enabled cam does not in all. Therefore, you can use the cheap USB cam engaged with a USB to Ethernet Converter (search E-Bay, ~5 USD/pcs), and the last one issue we still cannot overcome here is the need in an extra power path for the cam.
Then, as you can see, your particular problem has a solution.
Why Ethernet can...
RJR in his answer shows several technical reasons i find spurious ("sophistication", "dma-less", "cost") but it's not the case, and secondary --- it's the case.
The primary reason is simple and it is... (ta-da:-) the power necessary to enable the link.
Look up at the numbers:
100BASE-TX: Micrel KS8041 PHY (with xformer) ....
for about 200 Mbps (both dirs), consumes about 0.33 W, i.e. ~3 mW/m @ 100 m span
1000BASE-T: Micrel KS9021 PHY (with xformer) ....
for about 2 Gbps (both dirs), consumes about 1.12 W, i.e. ~10 mW/m @ 100 m span
USB 2.0: FTDIChip FT232R IC (self feed only) ....
for about 480 Mbps (both dirs), consumes about 0.08 W, i.e. ~15 mW/m @ 5 m span
G.SHDLS: Infinion SOCRATES IC (with hibrid) ....
for up to 4 Mbps (both dirs), consumes about 2.00 W, i.e. ~2 mW/m @ 1000 m span
How could this be interpreted? I prefer that:
if you want a faster speed, you need more power,
if you want a longer distance, you need more power too,
if you want a wider SNR margin, you need more power again,
- in any way, keep your technology power efficient.
In other words: Why USB wins table-long distances? because it's inefficient to waste 4(12) times more power when possible to work with only 0.08W. And why Ethernet wins building-long distances? because again, it's inefficient to waste 5(2) times more power when possible to work at only 0.25(2.5) of initial speed.
All other reasons, if any, are only and only secondary.
P.S. For that my opinion about RJR's tech reasons' "spuriosity" not to be naked, i'll promise to (try to) describe that informatively as i can if anybody addresses a separate question about it (i would not explain it here because it's far out of the scope of the OP's question).
P.P.S. Also, as i found my previous answer downvoted, i think i had not explain the similarity between 1000BASE-T and xDSL clear enough to be understandable by a side reviewer. Therefore if also anybody asks a separate question about that similarity, i promise to (try to) answer it too.