Why not more generally - allow let/var declarations in expressions?

coming from a long and rich C background, I have no issues with the
existing mechanisms... but for those languages that do support variable
declarations in for loops; I've always wondered why not any expression?

if( let a = ( let b = 10 ) * 3 > 10 )
... or ...

c = (let a = b*d)

granted, the scope is extremely limited in the last case...
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Why not more generally - allow let/var declarations in expressions?

coming from a long and rich C background, I have no issues with the
existing mechanisms... but for those languages that do support variable
declarations in for loops; I've always wondered why not any expression?

```js
if( let a = ( let b = 10 ) * 3 > 10 )
```

... or ...

```js
c = (let a = b*d)
```

granted, the scope is extremely limited in the last case...