The immune system is very adapt in recognizing foreign biological matter like bacteria, viruses and even another eucaryote organism's cells.

On the other hand, the body selects it's own immune cells for low responsiveness to its own proteins to prevent autoimmunity.

Cancer is essentially constituted of a body's very own cells gone aberrant. That means, these cells usually share several of the following characteristics:

• Internally and/or externally unregulated growth, proliferation, and expansion
• Loss of tissue function
• Migration to neighbouring tissues
• Denial of internal and/or external apoptotic stimuli (self-destruction)
• Evasion of immune cell recognition
• Inhibition of immune cell signalling

To break it down in terms of your question: cancer cells are naturally less likely to be targeted by immune cells than external pathogens, as they are basically a body's own cells. Immune cells, nevertheless, will kill wildly aberrant cells rapidly. That basically means cancer cells are naturally selected for variants that circumvent this line of defense. Either, they lose receptors by which they are primarily recognized by immune-cells, or gain/upregulate mechanisms by which they suppress immune cell-responses despite proper recognition.

Now, mRNA vaccines can reverse these effects by different mechanisms. You could potentially use them to

• increase the expression of receptors the immune cells use naturally
• decrease the expression of receptors inhibiting immune cell response
• introduce new epitopes the body knows how to react to, like surface proteins of a bacterium

All of these increase efficiency, efficacy and precision of the immune cell response against the targeted tumor cell.