Grammatically, "oneThing o anotherThing to suru" is not just "to make oneThing anotherThing," but also "regard/hold oneThing (as) anotherThing."
According to this construction, 必要とする is literaly "to regard something as necessary." It is a periphrastic way of saying "to need something." I don't know why Japaneae resorts to periphrasis in this case, but it may be related to the fact that 必要 has a derived adjective, 必要な.
Hmmm, 必要にする is awkward but this seems to be an arbitrary thing. It is equally natural to say 必要になる instead of 必要となる. In fact, 必要になる is more up-to-date while となる is more formal.
A similar expression is used with either postpoaition; 頼みとする and 頼みにする (albeit the latter is more frequent). This goes to show that grammatically both are possible for 必要, and that the reason has to be sought elsewhere as to why 必要にする is not used.
I sheepishly Googled "必要にする" and got hits of web pages where the expression is used to mean "set up something a mandatory condition" in the context of using software such as iOS and Oracle. Apparently, this is a very new development but this is my wit's end.