How many of those places have you been to? They might not need year-round A/C like some other countries, but the increasingly-common heat waves definitely require them. The buses are almost intolerable with air conditioning, there's no way in hell they'd ever purchase them without it.

The additional purchase cost is a rounding error, and you're far worse off if cooking people alive during the summer means losing customers year-round as they switch to less-hostile transit options. Maintenance isn't a dealbreaker either: sure, it's extra work, but the equipment is rarely needed. This means the occasional breakage isn't a huge deal, and big maintenance can be deferred to the spring and fall.