position of subject in nonfinite clauses

[J. Clancy Clements (Kapil)]

I have another question which I can't figure out a functionalist-oriented
response to.  In Spanish, nonfinite clauses (i.e. infinitival and
gerundive clauses) cannot have a subject preverbally.   For example,

Por decir estas cosas mi abuela...
because-of say these things my grandmother
"Because of my grandmother saying these things..."

*Por mi abuela decir estas cosas...

Any ideas about how to account for this would be most appreciated.

Thanks,

Clancy