It's not about the topic, but mostly the technique. You have to record and process a few hours of human speech in a studio (when I was there the "studio" was a closet with blankets on the wall). Most of the words are not cut in the same way as others text-to-speech engine would do, to preserve intonation, hence longer recording times and a "grammar" limited to the the span of the recorded vocabulary (see if you can catch how the topic is always about the rain. Maybe it was due to us being in britany, a very rainy french region)

However, given the number of complaints here, I don't think a cap on the number of topics she can discuss would be an issue for Alexa.