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Why were the canals of Mars considered an optical illusion?

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Mars
Mars An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.

The "canals" of Mars were not actual artificial structures but were optical illusions resulting from the limitations of early telescopic observations.12

When astronomers attempted to view the surface of Mars at the very limit of their telescopes' resolution, the human eye tended to connect disconnected natural surface featuresā€”such as crater chains, mountain ridges, and terrain boundariesā€”into perceived straight-line networks. Experiments conducted with untrained observers confirmed that such fragmented features are often misinterpreted as continuous lines when viewed from a distance.13

While some observers, most notably Percival Lowell, interpreted these markings as irrigation canals built by intelligent beings, the controversy was definitively settled by spacecraft imagery. Missions beginning with Mariner 4 in 1965 revealed that the Martian surface contains craters and natural geological formations but lacks any evidence of the long, straight, artificial channels previously reported.12