Why doesn't Manipulated Virus for Cancer Cure Work
This is not an idea I'm trying to make, obviously, I just couldn't get it off my mind and wanted to see what is wrong and if it is fixable.


Since everybody else is giving you a bad attitude in the comments...

While it might work for some cancers, it is completely impractical for the majority of cancers.

SOME cancers do occur because of a specific mutation or specific set of mutations. Some genes predispose people to greater risk for certain cancers. These have a much greater chance of being treatable via viral therapy, but there are still barriers:


The length of coding material (DNA or RNA) a retrovirus can hold and propagate is limited. The mutated gene or stretch of DNA may be much larger than the length a retrovirus could potentially replace.
The retroviral material is not guaranteed to "rewrite" or "overwrite" any bad DNA. Retroviral DNA is inserted fairly randomly into the host genome with little regard for where since it is generally an entire gene meant to be read in its entirety to eventually produce proteins. You'd expect BOTH gene versions to exist simultaneously. This wouldn't end in a cure, but some middleground between cured and affected. 
There are tissues where cancers develop that wouldn't be infected. Any sort of brain tumor would remain completely untouched by retroviral therapy, for example.
Targeting specific cancerous tissues while leaving surrounding or other tissues alone is a big problem, even for non-retroviral therapies. Replacing bad genes in a cancerous cell is good, but replacing good genes in a healthy cell could lead to that cell's death or -- ironically -- possibly turn it into a cancerous cell as well.


The majority of cancers are not specific mutations and do not have known genes that predispose their bearers. The majority are an accumulation of mutations across sections of the genome. As MattDMo said in the comments (paraphrased): For these cancers, viral therapy is nearly impossible. Every mutated gene would required its own retrovirus. At that point, the time and financial investments start to rise exponentially. 

While I'm not one to say it's impossible (I'm not involved in the research, so if anybody who is can correct me on anything in this post, I'd appreciate it) -- it is certainly very improbable with current knowledge and methods. Every bullet point above (which isn't an exhaustive list) represents a mountain to climb on hands and knees before an "inject-and-forget" end product could be developed.

Kudos for the critical thinking and forming a working theory from the knowledge you had, but unfortunately it does break down with some more specialized information.