Why is carbon-14 dating not accurate for estimating the age of materials more than 50 000 years old?

Its has a half-life of about 5,730 years. The short half-life of carbon-14 means its cannot be used to date extremely old fossils. Levels of carbon-14 become difficult to measure and compare after about 50,000 years (between 8 and 9 half lives; where 1% of the original carbon-14 remains undecayed).

Why is carbon dating only accurate with samples that are 50000 years old or younger?

Because of the short length of the carbon-14 half-life, carbon dating is only accurate for items that are thousands to tens of thousands of years old. Most rocks of interest are much older than this. Geologists must therefore use elements with longer half-lives.