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I'm not sure there's a technical reason, except usually the offered parts are multi-topology and then they could be listed as buck-boost while they at the same time could as well be used as flyback, SEPIC etc. 

This seems to be the case for TI and Maxim, which at a glance seem to call everything "buck/boost". Whereas AD, who's probably still the market leader, seems to list them separately and offer roughly as many buck/boost parts as flyback or SEPIC. 

In the case of catalog companies specifically, they might have many reasons why they only list certain parts. Most notably the ever changing, ever tiresome "linecards" of silicon vendors that they are allowed to sell from.

For example LT went completely bonkers some 10 years ago and decided that they would only sell their parts through Arrow. Didn't work out too well for them... Digikey & others would probably have liked to sell their parts, but they weren't allowed to.