A number of points:
1 It's in a section marked Refererence and not meant to be a list to learn, any more than a dictionary is meant to be a list of words to learn.
2 It's hard to determine the most common- what sources are to be used? There are many databases of language and they do give different results.
As a simple example, there are hundreds of postings on the web saying that nobody says 'it's raining cats and dogs' nowadays, so a simple search on the web for frequency would produce a result quite the opposite from that intended by those writers. A simple search for 'cats and dogs' (excluding any mention of rain) brings 246 mentions in a 400,000,000 (AmE) word database and 43 in a 100,000,000 (BrE) word one, suggesting that this is not a particularly common usage, but Google racks up 214,00 for a search for "raining cats and dogs". Who's right?
3 The most universally understood are actually probably among the least relevant in a reference section because they will be the least looked up. And again, this doesn't prove utility- if I come across an idiom in a text and need the meaning of it, finding the meaning one that one occasion is useful.
4 There is a degree of filtering available by variant, though not by region.
5 The idea of producing a list of the most common as a separate page is fine, but which would you suggest?
- For Teachers