Because that's how progress is made, and how markets stay competitive. By challenging the market leader.
Generally, though the idea is to challenge the market leader by producing a
better product - especially when you're a big international company with a shedload of money and a reputation for attention to detail.
For the most part I like it. I'm in a major United States city for the time being.
Yes, I think the complaints are mainly coming from those of us who
aren't in major US Cities and have suddenly found the maps less detailed and harder to read, our half-decent satellite/ariel views replaced by horrible fuzzy murk and the public transport feature replaced by 3rd-party plugins that don't exist, don't cover our area, have one-star reviews and/or cost serious money.
The UK mapping is absolutely pathetic compared to the old App - less detailed, harder to read, doesn't use the conventional OS colour coding for roads (the old App did) and minor roads are shown as white on a light cream background. The satellite ranges from
almost, but not quite as good as Google to bloody awful.
There are plenty of example comparisons online and, from my experience these are
typical - not cherry-picked examples.
Oh, plus, for the millions of people with WiFi-only iPads, gaining turn-by-turn is not a great advantage (and there were free turn-by-turn apps, anyway) - certainly not worth losing street view and public transport for.
(Did I mention I know one guy who liked the hocky-puck mouse, and another who liked Windows Vista?)