I understand what a matrix clause is, but was curious why it's called a "matrix" clause.
If you understand what matrix clause is, the way I do :
One clause may be embedded within another, that is, it may be used as a constituent part of another clause. Such a clause is called an embedded clause (or a subordinate clause) and the clause within which it is embedded is called the matrix clause.
This resembles one of the meaning of matrix which is also used to refer mass of fine-grained rock in which gems, crystals, or fossils are embedded.
e.g: nodules of secondary limestone set in a matrix of porous dolomite.
P.S : comments by @jlawler and @Xophmeister indicate almost the same.