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.
The windows often contain labels like "Fahrzeug klimatisiert" (Vehicle is air conditioned) so it is not that people are unaware.