For most large-scale manufacturers, the percentage of instrument-grade wood that you'll find in a typical log is a major consideration in species preference. Rosewoods, mahogany, sapele and bubinga tend to be quite uniform, with hard, dense wood throughout. Hardness tends to be more variable in temperate tonewoods than in tropical ones...from one log to another and even within the same log. While it is certainly possible to make an excellent guitar out of maple, pear or walnut, for example, it might be less than 5% of a large load of logs that will include wood with real luthier-grade potential, particularly if figured wood is wanted. TYPICAL flitches of these woods are likely to be of uninspiring character, and the large number of instruments made with these lesser grades no doubt suppresses the overall reputation for quality that these woods carry. Much of this softer wood comes from second-growth forests. With old-growth maple becoming rare and protected, it's not surprising that guitar makers are looking for more reliable and abundant sources of top-grade supply.
We should also remember that "maple" is a genus (Acer), not a species, and the hardness range runs from quite soft (Acer macrophyllum, for example, with an average Janka rating of 850) to nearly twice that hard in the best examples of Acer saccharinum or Acer pseudoplatanus. Nice tonewood...if you can get your hands on superior pieces of it.