Pennsylvania was one of the slower states to adopt the "no-fault" divorce standard, which had been gaining traction since the 1940's. No-fault means that there is no requirement to show that the divorce is the fault of one party of the other. Common fault grounds include adultery, abandonment, cruel and inhumane treatment. Fault divorces can be nasty, contested, and expensive affairs. The no-fault standard eliminates these long and drawn-out legal battles and makes the process relatively simple. In many cases, the entire process need not take much longer than 3 months. The reason for the divorce generally has no impact on property distribution so even if the parties are fighting about how to split up their possessions claiming that one person is at fault won't have an affect.
Those who wish to can still file under fault grounds, but as mentioned it's often much more expensive and complicated. There may be reasons a person wants to file under fault grounds, for example religious persons concerned with how they will be treated by their organization may want to prove they are not the one responsible for the divorce. Getting an annulment often requires some fault ground. There may be familial issues at play as well.
Anyone interested in a fault or no-fault divorce should also consider the filing fees, which can vary considerably from county to county.