Then, why has it been so hard to turn the Theory of evolution into Law?
Because the term "law" isn't used any more is the biggest reason. It is an old fashioned term.
In addition, if you look at what we do call laws, they are more focussed and much, much less encompassing than modern theories are.
Newton had his "laws" of motion (actually without any real "theory" explicitly stated). They are simply equations describing the way bodies move. Einstein has his equations describing the same thing but they are not called laws they are embedded in the two theories of relativity and the theories make broader statments than just those equations. We will probably never call the equations of relativity "laws" even though they are more accurate than Newton's equations.
I suppose one could liken some population genetics equations to Newton's laws of motion and call them population "laws" but, as noted, that isn't done any more. The theory itself if much more encompassing than any such details anyway.