Because more people will choose to ride the public transportation at lower cost.
Some one must have done the survey to see how many more passengers they'll get from the lowered price. It seems reasonable to increase the demand by lowering the price, but with much higher volumes.
The break even point is 50% more ridership. If they have 10,000 riders at $1.50 ($15,000 gross revenue), they'll need 15,000 riders to break even at $1. The likelihood that it'll increase by 50% is the issue. If they increase ridership by 51%, their net income increases by $150. If ridership increases by 60% (or 10% more than break even), their revenue increases by $15,000.
This only looks at the revenue side, but the company must also consider any added cost.