You are actually asking why our RINO congress critters are spineless and torpid in the face of 'rat depredations? Seriously? Why is water wet? Why are our borders wide open 3 years after 9-11? Etc.
1. The GOP is doing their anti-fraud work by stealth, and don't want anyone to know about it.
2. The GOP is suicidal.
3. The extent of voter fraud is wildly exaggerated on forums like this -- and it is really not enough for the GOP to consider it worth the cost of dealing firmly with it.