Because its set up in such a way where 300 was chosen as the highest score they would award, but because the score given is based on how many SDs from the mean you are: 300 has not been an attainable number for a long time/ever. The "300 point" would require a SD that implies you got more questions right than there were on the test. If the SD were to tighten then hitting, and perhaps even surpassing, that point that would be possible, as its all SD-derived. In that case anyone above that "point" could only earn a max of 300. But again, because of the way the test is currently calibrated, 300 is not a possible score, though it is the theoretical maximum they'd ever award.
Think of it like the 99. How come the 99 point is so low that a decent portion (15%?) of people get it. Why don't they raise whats a 99. Because 99, where it is, was selected by some esoteric method as the mathematically most appropriate place to put it. It's been getting easier and easier to get in recent years as a 232 was a 99 this year, but the math behind it (I have no clue what it is) is unchanging, the score demographics are what changes and makes it more common. The 300 is the same way except the math behind it ended up putting it outside of possibility in the current, and supposedly any previous, score demographics.
tl;dr: blame mathematicians. They ignored reality so that their math would work out more 'neatly'
