She probably just wanted to? She grew up with a lot of siblings and half-siblings. Most of these siblings and half-siblings have a bunch of kids, too. Two of her older sisters have four children each. Maybe Kylie Jenner just wanted to have a family early so her children could grow up close in age to their cousins.
Of all the things you could dislike the Kardashian-Jenner family for, their decision to have a bunch of children is the least worthy of hatred and derision… it’s a very normal desire for people to have. And sure, you can whine about her being “too young” but why? She was 21 when she had her first kid and she was already a multi-millionaire at the time. Her makeup line and the royalties from her appearances on reality TV had already given her enough money for herself and any potential child she’d have to never have to work a day in their lives and still be comfortable for several more generations…
There’s no such thing as “too young” or “a bad timing” for having a family when you’re fabulously wealthy — I mean sure, if she had been a teenage mother I would have bought the “too young” argument but a 21–year-old worth 100 million can do pretty much whatever the hell she wants and it won’t negatively affect her quality of life in any way, shape or form.