Megillah 3a - Megillah Reading vs. Torah Study

One should be mevatel talmud torah in order to read the megillah. The Maharatz Chiyus asks why is megillah reading referred to as bitul torah, isn't the megillah also torah? He answers that the travel to hear the megillah read in a minyan is bitul torah. This seems difficult to me in light of the gemara in chagiga 5b that one who learns torah for just one day a year (and spends the rest of the time travelling) is considered to have learned the entire year. Clearly, travel time to study torah is not counted as bitul torah. It seems more plausible to me to answer based on the gemara in chagigah 10a that there is bitul torah in quality, not just in quantity. To go from gemara to mikrah is bitul torah in quality (see also baba metziah 33a).

However, the gemara seems to give the impression that being mevatel torah for megillah is the exception, not the rule. We are always mevatel torah for mitzvos that can't be done by others (moed katan), so why does the gemara make such a big deal the we are mevatel talmud torah for megillah reading?