No, it’s because LA’s subway system is relatively new and, since subways are so expensive to build in the US, not very extensively built out. To be honest, it’s a miracle that LA has a subway system at all, given how irrelevant it is to most people in the city.
The Blue line opened 30 years ago in 1990 and the most recent line ,the Gold Line opened 17 years ago. The four other lines opened between those years. It's only new relative to say New York's subway or London's but it's not really new to Angelinos at this point.
but construction hasn’t stopped. the expo line for instance opened in 2012 and was extended to santa monica in 2016. the purple line extension and the crenshaw line, to name a couple more, are highly anticipated and ongoing as well.
Philadelphia hasn't really expanded its subway system since the 1950s as far as I can tell. Only a few renovations of certain stretches of track (much of which is actually elevated, not underground). Even then, its not a very extensive system; just a north<->south line and a northeast<->west line.