There are several factors contributing to the current global helium shortage. There are many people who believe the balloon industry is the reason for the shortage and the party industry does account for a lot of the global helium use. However, the main reason is due to depleted resources and a decline in production.
Helium is used in many different industries and businesses, not just the balloon industry. It can reach extremely low temperatures without freezing making it very useful. For example, industries such as aerospace need helium to clean fuel tanks on rockets, the technology sector uses helium in optical fibres to bring internet to people, and the medical field needs helium to operate MRIs.
This non-renewable vital natural resource is found in underground chambers and reservoirs. The gas comes from the radioactive decay of uranium which causes helium to be released into underground gas chambers and reserves over millions of years. Oil companies then extract the helium through a process called fractional distillation. This is when the natural gases are broken down into their individual elements in order to isolate the helium.
See What is helium? for more information.