So, anyway, is there an official reason why a story that takes players to level 20 wasn't pursued?
Officialy, the original plan was level 10 as a cap ...
Then they find out that 10 is not enough, so they needed to raise it "a little" ...
Thats all what was said, anything beyond that is just speculations.
It is almost certainly to leave room for a sequel/expansion or separate story using the engine. Level 10 is the cutoff for 'tier 3' officially, but 12 is the last level before you hit the really insane spells and get the next proficiency bonus (from +4 to +5) so it is actually a good soft cutoff for low-medium tier vs high tier. That seems to be something that they had to work out with WoTC though, but I think the long development time table may have broken some of the tie in plans and given them some more breathing room.
Regardless, it is likely that there will be more content after this if it sells well enough. Whatever agreement they have with WoTC is going to dictate what comes next, so without seeing that contract, there isn't a lot of solid guesses we can make. At the end of the day, money makes the world go round, and WoTC has taken some hits lately, so any financial 'sure things' are going to make big daddy HASBRO push to recreate. Larian, as great as they are, have been close to shutting down after almost every one of their releases, so I'm betting they would be happy to make an expansion or sequel if the financials look good and they are allowed to.
There is a great History of Larian video out on youtube somewhere that is worth watching if you're interested in learning more about the company.
