The main causes are they use different frequency ranges and the signal strength is supplied from the sending end.
On ADSLx the upstream frequencies are the low ones, and a narrow range. These attenuate (lose signal strength) gradually. The downstream frequencies are immediately above the upstream, and higher frequencies attenuate faster. Which is why the speeds fall off at greater distances as the highest frequencies become unusable.
On FTTC the principle is the same though the frequencies come in a number of groups alternating so it works a bit differently. However, VDSL2 signals attenuate much faster than ADSLx ones - I don't recall the ranges but of course the larger number of high ones cause a faster drop off with distance.
Which is why over a few hundred metres FTTC is so fast, but at long distances is no better and often worse than ADSLx.
Very broad brush there with a lot of detail missing, but I hope gives the general idea.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
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