My understanding it is very linked to 1) Profitability. Search terms around things like lawyers and credit cards. You'll almost always see 4 ads. 2) Genuine rel

My understanding it is very linked to 1) Profitability. Search terms around things like lawyers and credit cards. You'll almost always see 4 ads. 2) Genuine relevance. Google know for certain searches your not likely looking to buy something and to keep credibility don't show ads. Occasionally you can find pockets of less competitive search's that 1) allow ads 2) relate to your product via the algorithm even if they don't to a human brain 3) Align to your desired audience and these can give great return.