Earlier quoted context omitted.
I haven't seen a non-AC bus in ages, even in developing countries.
Buses in places like Ireland, Scotland, much of Scandinavia, etc will never need air conditioning. Places a little warmer (England, Denmark, Netherlands, northern Germany) might be warm enough for a few days per year, but the cost of purchase and maintenance of A/C might not be worthwhile.
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.