It is probably several reasons. Like cost, and weight reduction, the comparable litle gain for most users going 300 bar. Also the 250 bar bottles work for all FX guns, regardless of having 1, or two regs.
If you use a scuba bottle as fill source, only a few fills will be able to be over 300 bar, while most will be in the 250-170 bar (roughly). As if you fill your bottle at a fire station, you will most likely refill at at 200-170 bar.
You would gain more by having a bigger 250 bar bottle, than to refill a 300 bar fill source more often.