The Philadelphia Union's match against FC Dallas being postponed due to snow was unfortunate, yet not totally unexpected. It's not the first time that an MLS match has been postponed due to snow, nor was it even the first that the Union were playing in that had to be held off due to it. Their match against the Colorado Rapids back in 2013 was postponed a day thanks to snow in Denver. A playoff game in 2012 between D.C. United and the New York Red Bulls was also postponed thanks to snow - this after the matches had already been rescheduled due to Superstorm Sandy.
The issue here though isn't the weather. There's a chance of snow in most MLS markets in the early parts of the year as well as around the playoffs. The issue is that by and large, MLS stadiums are not designed to handle snowfall.
PPL Park is a prime example. While some MLS stadiums such as Chicago's Toyota Park, Sporting Kansas City's Sporting Park, and Toronto's BMO Field have undersoil heating - a collection of heating coils under the surface to help keep snow from accumulating on the pitch - others such as Red Bull Arena and Montreal's Stade Saputo do not. PPL Park does not have this, so any snowfall is kept off of the pitch by tarps that are laid out on top of it. The snow is then plowed off the tarps and the tarps are removed after any accumulation, thus making play during a snowstorm impossible.
Another disadvantage to PPL is its roofs - more specifically the lack thereof for both the River End and Chester End. Approximately 4,000 to 5,000 seats have no covering from the snow, and large portions of the River End have metal flooring, which gets slick even during a rain storm much less during a significant snow or ice event. There's the potential there that people going up to or down from their seats could get seriously injured from slipping and falling on this surface.
PPL also doesn't have many wind breaks. While the Commodore Barry Bridge and the off-ramp to Seaport Drive offer a bit of a break to the northern and eastern sides, there is not much of a break to the west, where the vast expanses of Lots B and C offer zero resistance, or the south, where the Delaware River flows. On either side of the River End, there are open corridors where you can see the river from the pitch.
Are these things that should have been taken into consideration when PPL was being built? Perhaps, but MLS has been adamant that they are not going to the FIFA calendar. It may have also increased the cost to build the stadium enough where it just wasn't economically feasible to build for something needed only once or twice a year. It also may have come down to timing - PPL wasn't opened until late June in 2010, and adding these features on may have pushed the opening day back even further into 2010. Perhaps as the inevitable stadium expansions happen, these accommodations will be made. But until then, it's unlikely that the Union will play in the snow.