This is a spin-off question from this one "[Determine radicals for simplified characters that lost their traditional form completely][1]" where radicals for simplified characters were discussed.

Now, the question is, how were some characters simplified, sometimes so simplified they lost their original intention and form completely?

One example is:

葉 (leaf) became 叶, where 叶 is basically a mouth (口) and a ten (十) put together.

What's the rationale behind this??<br/>
And why were some complex characters not simplified at all?

For example:

(traditional : simplified : meaning)

單 : 单 : single, also a family name<br/>
嚴 : 严 : strict, also a family name<br/>
嚼 : 嚼 : to chew (here traditional and simplified are identical and yet it's complicated to write)

Note that the two 口 on the top of 單 and 嚴 were transformed into different forms when simplified.

  [1]: https://chinese.stackexchange.com/questions/282/determine-radicals-for-simplifed-characters-that-lost-their-traditional-form-com