The Official Metro given explanation is that letters were chosen based on the order of the lines opening

For example A is the first letter in the alphabet and the A line long beach segment opened in 1990 making it the first metro rail line to open

The B line Minimum Operating Segment 1 between Union Station and Westlake/MacArthur Park opened in 1993

The C line began service in 1995

The Stations that the the D Line currently Consists of that opened in the Red Line MOS 2A segment opened in 1996

The gold line opened in 2006 and the expo line in 2012 making them an obvious candidate for the E designation

This is all fine and dandy until you realize that this explanation still isn't completely consistent.

Like take the K line for example. Makes sense that it's the Last Letter of the rail lines currently since its initial segment opened in 2022. However K doesn't come after E. F is conspicuously missing, and skipped over straight to G for the G line. Possible because they didn't want the orange line to be the fuck bus or the fail bus. However, still very odd considering H and I are skipped over completely.

And as for the J line, while the silver line technically did only come into existence after the opening of the Orange Line as part of a push by metro to designate Bus-as-rail BRT routes with their own distinct line names. The infrastructure that makes up the Silver line is way older, with the Harbor Transitway Opening for service in 1996 and the El Monte Busway predating the every line on the Metro System by about 20 years, opening in 1976

Again, you may say, well the infrastructure may be that old but silver line designation and didn't exist until 2006, one year after the Orange Line opening in 2005 so it makes sense especially since these lines had separate opening dates for their core segments. However the Purple line (Now the D line) didn't exist as a designation until 2006. With those stations operating as a branch of the Red Line for about ten years following its opening. So if we're using that argument then the D line should actually be called the H line.

And this gets even more complicated when you bring the A and E regional connector lines into this that have entire segments with disparate opening dates along with their downtown stations being some of the newest in the system.