People won't buy your next game if they don't enjoy the last one.
If people didn't finish your last game, then chances are that they weren't enjoying it.
Therefore, you want your audience to be able to beat your game.
Unless you're running a niche franchise like Dark Souls, which has a niche fanbase that really wants very hard games, you're going to be designing your games around the lowest common denominator. Ie, a noob, someone who doesn't hardcore plays games over and over, who perhaps only buys one game a year, or perhaps have never bought a game before. Therefore, the game needs to be easy enough to be able to be beaten by everyone.
The developers of The Banner Saga learned this lesson the hard way. Their game is a story focused SRPG/TRPG. Basically, Fire Emblem. You had SRPG players backing it on kickstarted, but the average person who bought the game wasn't a hardcore SRPG player. They were just a casual, who either wanted a good story, or liked the artstyle. When the game first released in 2014, the final battle was incredibly challenging. I game overed twice on it, and I really enjoyed optimizing my team and coming up with a strategy and carefully making my moves, and the satisfaction of finally achieving victory. Really felt you were overcoming against all odds.
However, half of the playerbase dropped the game at that point. That's a huge amount of unsatisfied customers who will probably not buy the sequels. Which means that's a huge financial loss.
Link to interview
The devs heavily nerfed the final battle, and while a lot of hardcore players like me complained, in the end was the right business decision and completion rates went up, and more people were inclined to buy the sequels. After TBS1, all of the battles were designed to be easily beatable. For us hardcore SRPG players, sure you can try playing on hard, which adds a few more units to each battle and slightly bumps up the numbers, but there is no getting around the fact that games are simply designed to be beaten by everyone. It's the right business decision.