Because the transit authority has to work at taking your fare. Vs. the easy route of trusting a cheap & convenient app ecosystem (helpfully run by SpyCo, CreepTech, and DataHarvest Ltd.) do all the hard bits. While the authority basks in feeling cool and trendy. Plus, smartphone fare fraud looks really complicated (vs. counterfeit metal tokens or whatever) - so the authority feels no pressure to do anything about it.
Less cynically, a good national policy might be to treat all "excess" revenues collected by organizations providing better deals to those passing various wealth/class/fluency tests ("has bank account", "has smart phone", etc.) as "poor taxes". Which are totally legal...but must be accounted for in excruciating detail, and every last farthing turned over to the greedy nitpickers staffing the Poor Tax Dept. of His Majesty's Revenue Authority. Every month, no excuses. So, to avoid that monthly misery for their so-privileged selves, the managers at every org with the paper power to collect poor taxes work hard to make sure that they are not doing so.
Less cynically, a good national policy might be to treat all "excess" revenues collected by organizations providing better deals to those passing various wealth/class/fluency tests ("has bank account", "has smart phone", etc.) as "poor taxes". Which are totally legal...but must be accounted for in excruciating detail, and every last farthing turned over to the greedy nitpickers staffing the Poor Tax Dept. of His Majesty's Revenue Authority. Every month, no excuses. So, to avoid that monthly misery for their so-privileged selves, the managers at every org with the paper power to collect poor taxes work hard to make sure that they are not doing so.