When I search the Plant Database for Ilex verticillata, this is the result I receive:

http://garden.org/plants/searc...

Why aren't all the listings in alphabetical order - particularly the list of cultivars?

When you search for something on google.com, the results are not in alphabetical order. They are in "Best match" order. Generally speaking, the search that you are using here returns them the same way. The order is in what the search engine thinks most closely matches your search.

If you want to see them in order by cultivar, then you should use the browsing tools.

http://garden.org/plants/brows...

I used the browsing tool.

It seems the order of listing is solely based on when the plant was entered (plant ID number). This is constant for the search I linked to above for Ilex verticillata, as well as for searches I've done for Viburnum dentatum.

If this is the way it must be - so be it. If searches can be keyed to the plant name, though, it would seem that a search under Viburnum dentatum would yield the results in alphabetical order.