Why do they wait so long before executing the drug bust?

The headlines in the paper today read “Big drug bust hits area”. It was an international ring. It further says ”...the group had been under investigation for almost 15 months and was responsible for moving about 2 kilograms of the drug per month in the Rochester area.”
Why does law enforcement wait so long? According my math that means 30 kg of XYZ drug made it out into the community. That is 300,000 doses at about $10 each. $3,000,000! Lives were lost from OD, wealth was wasted, US cash was sent overseas to Chinese labs making the product, productivity decreased, relationships fell apart, crimes were likely committed to pay for the drug, property values decreased, people were hooked and became dependent, jobs were lost, etc.
What were they waiting for? Was there one piece of info that sealed the deal?