I didn't mean for you to have to take my word for the reasons courts have exercised self-restraint when it comes to reviewing the means selected by Congress. Consider this language from McCulloch v. Maryland (1819):
"Should Congress, in the execution of its powers, adopt measures which are prohibited by the constitution; or should Congress, under the pretext of executing its powers, pass laws for the accomplishment of objects not entrusted to the government; it would become the painful duty of this tribunal, should a case requiring such a decision come before it, to say that such an act was not the law of the land. But where the law is not prohibited, and is really calculated to effect any of the objects entrusted to the government, to undertake here to inquire into the degree of its necessity, would be to pass the line which circumscribes the judicial department, and to tread on legislative ground. This court disclaims all pretensions to such a power."
It's unclear to me why you feel that Congress lacked the power to suspend literacy tests in order to enforce the Fifteenth Amendment.
1) Is it because you are interpreting the word "enforce" so narrowly that it would not encompass the suspension of literacy tests?
2) Is it because you simply reject the factual determination by Congress that suspension of the literacy tests was "necessary" to enforce the Fifteenth Amendment?
3) Is it because you feel that suspension of the literacy tests raised federalism issues because it unduly intruded into areas (voting procedures) historically regulated for the most part by states?
I think your information is a little old. This was back in the days when it was considered next to unthinkable for SCOTUS to strike down any act of Congress at all (even though Marshall reluctantly allowed for the possibility). Yes they had done it once, but only in a case where the act ordered judges to do something that they themselves felt wasn't within their constitutional prerogatives, so Marbury vs. Madison, properly understood, was actually a case of judicial restraint, more than anything else. It wouldn't be until 1857 when SCOTUS would strike down another federal statute. Back then, many, as you pointed out, were of the opinion, however wrong, that the courts simply did not have that power. Nowadays, no such opinion would be taken seriously.
Also, in 1819, the courts weren't anywhere near as prolific and arbitrary in striking at state actions, and the phrase "compelling government interest" had not yet seen the light of day. My point stands that it would be totally inconsistent of the courts to have the attitude they have today towards the states, but continually walk on eggshells with regard to congressional actions.
Is it because you are interpreting the word "enforce" so narrowly that it would not encompass the suspension of literacy tests?
Enforcement does not encompass new general prohibitions on behavior, period.
Is it because you simply reject the factual determination by Congress that suspension of the literacy tests was "necessary" to enforce the Fifteenth Amendment?
I reject a priori the notion that one needs to prohibit additional activities in order to enforce existing prohibitions. There are time-honored procedures in place for enforcing laws, and for upping the ante when existing policies are ineffective at curbing violations. All Congress needs to do is follow them. Anything else is not necessary, and certainly not proper.
Is it because you feel that suspension of the literacy tests raised federalism issues because it unduly intruded into areas (voting procedures) historically regulated for the most part by states?
I think I'd be making the issue more complicated than it needs to be by framing it in those terms. It's because it involves Congress assuming new powers not granted to it, not merely adjuncts to existing powers. And because the line needs to be drawn somewhere, otherwise it would make meaningless the notion of a federal government with limited powers. They could justify any action as being necessary to the exercise of a particular granted power.