Because in development (games or otherwise) you have limited resources. Said resources should, and are, focused within the various industries on making products usable - or in the case of games like Bannerlord - playable.
Yes, they could indeed focus on feature adding, but if the game crashes frequently and saves are corrupted and lost permanently, then there isn't even a game to play (and likely countless refunds, making it increasingly more expensive to update and maintain). They're effectively fixing the framework and making what we've got right now more stable, which will give them - and modders - a better platform to build from.
Indeed they could abandon the game in it's current state with only a reputation backlash, if they wanted to, so we should all be grateful for the continued development, rather than whinging and whining that they aren't doing it the "right way". And come on people - those of us who are modders might be actively creating content, but there are probably already more modders of one flavour or another than developers - and we can do whatever we want without restriction, and likely enjoy it a damn sight more than they. If I spend 8 hours trying to get a relatively simple feature done, it hasn't cost them anything. If a dev stays on past 8 hours, he needs to be paid.
Say this. If you're a prospective buyer, and you see one game with A and another with B. Which are you more forgiving of and more likely to buy?
A. Reviewers say the game is fun, runs fairly well other than the odd hiccup, but feel the game is lacking content. You notice it states early access, and is in development.
B. Reviewers say they cannot play the game at all - it crashes constantly, hours of save progress is lost, and though it is updated frequently, the developers appear to be piling broken content on top, rather than making what exists playable.