To solve this, we'd need to know how the porch floor is constructed; is it wood, concrete slab, up on piers. Also, the structure could be fine, but if they did not install properly, that can cause problems - expansion joints are required, depending on sun exposure, size of the installation, and proper grouting to allow for expansion (there should be a gap against, for example, the house).
If they used cbu on top of a wooden subfloor, did they tape the seams, did they install the proper fasteners, did they install the cbu in a bed of thinset?
Normally, cracked tiles are the result of either improper structure (moves too much), or incomplete coverage of thinset, which therefore can't support the tile properly from point loads.
Also, you didn't mention if this was in an area that sees freezing temperatures. Improper slope so there is standing water (even though it is covered, it's not uncommon to get wet) during a freeze cycle could severely impact integrity.
A tile that can absorb moisture, will likely fracture during a freeze cycle, so drainage and proper installation is essential. Outside, people often find that a good quality porcelain stone look tile survives longer since it can't absorb much moisture (typically much less than 1%).