Why are there just nines in the seed and not a 1 or a 4 or a 6? Is this just picked randomly because they like the 9 or what is the reason behind it?
You want to have 27 unique symbols due to ternary computing. The Latin alphabet only provides 26. Some numbers like 0 or 1 might look like letters (O or L). Besides that they could have picked any other symbol that doesn't give confusion with letters.
Arabic numerals are available in every input schema so a number makes sense. Which one isn't that important.
Come_from_Beyond answered that on reddit:
There are 27 different tryte values which are represented as 26 letters of Latin alphabet + "9". "0" is too similar to "O" and may create issues for those who write seeds on stickies.
Source: Reddit Post
It's arbitrary really. A trit can have any 3 values. 0,1 or -1. Hence there are 3^3 = 27 possible tryte values. Easy Enough to denote 26 of of them by the letters of the alphabet.
So then the final question because 'what should we use to denote that last value'. I could have picked 7 and it'd have worked just as great.
But yeah, 9 works too.