Why is 走 distinguished from 辵?
The simplification has a practical purpose, that is to ease learning and daily use. 辵 is an archaic form that's not used in any modern context. There is no point simplifying it anyway. I doubt that the 简化字小组 ever looked beyond the 4000 most frequently used characters.

There are over 50,000 characters that have ever existed, only about 2000 get a simplified version in 《简化字总表》, if you don't count the radicals and derivatives, the simplified character count wouldn't exceed 400.

For the sake of argument, there could be a few other reasons.


They usually take an existing homophone character to substitute the complex one, 後 to 后 for example. 走 and 辵 sounds nothing similar.
There is this principle called 述而不作, which means they try to find existing simplifications and make them standard instead of inventing their own (though in many cases the simplified form is rather obscure). The 辵 to 走 simplification never existed.
辵 is already simple!