It's a supply issue from what I've read. Most bomb sniffing dogs are sourced from overseas and there's a high demand for them globally. There simply aren't enough dogs that are capable. Even from a litter, there's a chance none of them will qualify after training, much like legit service dogs. We also use a lot of dogs to sniff for drugs.
For drug-sniffing dogs, generally all that is needed or expected is the dog will indicate where and when its handler wants an indication. For bomb-sniffing dogs you are hoping for a more precise outcome which is much harder to train for and achieve.
>The ADE 651 is a fake bomb detector[1] produced by the British company Advanced Tactical Security & Communications Ltd (ATSC). Its manufacturer claimed it could detect bombs, guns, ammunition, and more from kilometers away. However, it was a scam, and the device was little more than a dowsing rod. The device was sold for up to US$60,000 each, despite costing almost nothing to produce.
