One influential promoter in a crime is the number of potential escape routes; the more the better.
An alleyway only has one, or possibly two escape routes, not making it a great location in which to commit a crime. On the other hand, a transit stop, which is a transport interchange, having multiple stations, platforms and stops, is like a rabbit warren, and there are many, many escape routes a criminal can take after committing their crime.
There are, of course, those things which can help prevent muggings and other crimes, such as security guards, security cameras, and even vigilant street vendors, etc. When we look at the locations where muggers operate, we can see that they are relatively busy places, with consistent foot traffic, and a good number of pedestrians.
So the question remains, how can they commit a crime in such a place without being spotted?
It is relatively easy for a mugger to slip in close behind a victim who is standing on a crowded escalator, and stick a knife in their back, whilst demanding their wallet, without their weapon being seen or their demand being heard.
Even if somebody on the escalator did recognize what was happening, it is unlikely that they would intervene, for fear of having to deal with an armed assailant, themselves. Once the mugger has the wallet, he can start walking up the escalator, using the people around him to disguise his exit. In such a situation, where would you throw the wallet?