Because your mother knit her baby's things with it, and her mother before her and her mother before her. Because as Elizabeth Zimmerman told us, "wool is good for a baby." Why? Because it's wool. Because it's natural, it's warm, it's soft and it has a feeling of comfort, history and home.
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As you can see, I am biased. I knit my baby his going home outfit (my own design) in 4-Ply Soft, as well as his baby Moc-A-Socks (oh so cute!) and his Penny Straker Baby Owl Sweater.
I have 3 bags of Cashmerino Baby in the stash and they have never been cracked open. Not that Cashsoft 4-Ply or Cashmerino are "bad" yarns, but they are not heirloom yarns. If I was going to knit for my own baby, something that would stand the test of time and be there for future generations, it wasn't going to be with a yarn that was part microfiber. Soft, yes. But they won't wear for 100 years. They just don't have it in them the way you did, 4-Ply Soft. For a close relative or friend, I would totally go for one of the 'cash-yarns'. It's a great yarn. It will be soft, warm and easy care. It's one thing if I shrink my baby's going home sweater (because I did, but let us never speak of it again) but if my sister had--well neither I nor she would ever forgive ourselves. As a knitter, I will miss this yarn. It was a beacon of softness, balanced with sturdy and gutsy construction and excellent stitch definition in a world of wimpy merino blends.
RIP Rowan 4-Ply Soft.
You will be dearly missed.
(If you still need a 100% wool baby yarn, may I recommend Dale Baby Ull?)