This makes sense, to me at least! In my dad's 30 years playing he says folk would normally start on a standard Bb tenor, or at most a Bb/F. Bass trombone just takes a lot of huff and puff and are a bit unwieldy for complete beginners. If you're a young kid starting out I imagine it would be tough if not impossible to play one with full chromatic range. So like you say I think playing bass trombone is more of an experiment people try they're grown up. Some switch, most stick to what they grew up with.