There are big differences in metals.

I'm guessing that the brittle stuff in the kit was either hardened but not annealed (i.e. it didn't have that gentle reheating step that sorta softens the temper a bit), or it was not properly annealed (there is a cost involved in the process controls that allow uniform product quality), or it may have been specified by strength only.

That's a real problem in how people (maybe including the people who put together your kit) think about metal products. They talk a lot about strength but may not pay the attention they should to stiffness, hardness, brittleness, and things like that. The wire in the kit may have been considerably stronger (i.e. able to bear more stress in the usual modes) that the LHS replacement even though it was unacceptable for its role because of being more brittle. On the other hand, you wouldn't believe how strong lead is, but it is also unsuitable because of how easily it deforms under stress.