Not just that everything was going through windows as GP said, whatever market they entered, they acted like their product will be like windows in that sector too from day one. Zune was like that, but the best example is windows phone, version 8 more precisely which is the first proper modern smartphone version.
Google realized that if they want to stand a chance in catching up to the iPhone, they need to shove android in people's faces, and lure in devs.
Microsoft entered the game (WP8) when android already had a foothold, making it even harder. They started with a mostly empty app store, and while they were clever enough to make sure the most widely used apps would be available by effectively bribing those big companies to develop windows phone apps, they pretty much gave the middle finger to all the small indie devs. I remember when android 2 was around I just downloaded android studio and played around a bit, making a simple scrobbler app for my Samsung device. Sideloadong was king back then, but even up to this point I had to pay zero bucks and jump through no hoops to try this out. I don't remember what putting this on the Google play store would've cost me back then, but not much.
The windows phone experience was: sign up for a dev account to download visual studio with WP support. Start up VS, asked for your account again. I think in the beginning this was actually a paid account, probably because apple did it that way and again, you're Microsoft so act like you already own the place. But later in they reversed course here at least and you could log in with a free account.
So you start building a small test app and then you want to run it in the shipped emulator but surprise! Your laptop only shipped with windows 8 home which doesn't include virtualization features, so tough. So the only way to test the app was to push it to your phone, which was another overly complicated mess where your phone had to be in developer mode and you could only "sideload" one app at a time, iirc. The result was an app store with mostly tumbleweed. Whatever small utility or gimmick you wanted, when on android a search would give you dozens of results, on WP, there was maybe 4, and 3 of them almost unusable and abandoned.
I'm not blaming ballmer for having decided this specifically, but holy hell how did this pass any meetings with the higher-ups? You're uo against two tech giants who have a head-start of a few years, you try to get people to switch to your platform by being pricey, having no apps, and being hostile to smaller devs?
The same played out with all the phone makers, who had to pay license fees for WP when android was free to use. Guess which phones were cheaper in the end. And when Microsoft bought Nokia, Nokia had the unfair advantage of getting WP for free, making it even less attractive for others to compete in that sector.
And let's not get into the botched Nokia acquisition because I also don't think this can be blamed on ballmer that easily, or primarily.