The UV light component of natural sun light, tends to break down most materials. So only tarp material of a certain thickness can be expected to be able to withstand slowly getting scorched by the UV of sunlight.
Even tarp material that is classified as UV resistant , will suffer degradation by UV ligt, it will just be slower.
It is also very difficult to make a completely waterproof seem, if using stitching (read: a sowing machine). Which is why tarps that are (and will remain) waterproof are typically melted together using ultrasound. As ultrasound melting can be done in such a way the the heat happens right between the two metal lips of the ultra sound joinery machine.
These aspects typically makes durable and long-term waterproof tarps a bit spendy.