Because yx=expr means "evaluate expr and set the value of y to it". If expr contains evaluatable subexpressions, they will be evaluated. Here, you're setting P after evaluating the expression that contains it. If P already had a value when you evaluated it in setting yx, changing its value will have no effect on yx.
For parameters like this, I like to use rules to define parameter sets. Leave P, EI, and L as undefined symbols, and define a parameter set as:
ps1= {EI->6, L->1000, P->100};
Then, at the point where you're ready to make the substitution of parameters for symbols, write expressions like yx/.ps1.
