Another reason may be that the coinage of the restored empire (1261-1453) was in many ways much more complex iconographically than that of all the preceding centuries, with a proliferation of types, but with examples often not well struck up. However, I also noticed numerous lacunae in the coinage from the earlier periods as well, but perhaps it is a work in progress. And it was fun to see that some of the coins from my modest collection were chosen for the photographs.
It certainly doesn't have all the coins as photographs aren't publicly available for some of them. However, it does cover coins beyond SB2020. You need to change the search options to be coins besides "Byzantine". Edit the field and once you do, you will be given several options including the "Latin Empire". The other fields are also editable. Remove "Byzantine" and make it blank and you get all the coins that are currently available. They also have some of my coins as well and ones that I got outbid on too :-(