Historical inertia, with a declining market. MS know they need to do something, they just fail to be cool.
As per previous peoples comments, MS DO get the market, just not the same market (necessarily) as Apple. Apple chased and won over the consumer with the iPod, then the iPhone then the iPad and now the ecosystem is drawing people into the Mac/OS X world as well. Yes the Mac has its place, predominantly with App developers (more recently 8-9 years) and creative professionals etc.... Windows has on the most part ALWAYS dominated the Enterprise and professional business sectors, because of its flexibility to run on a large variety of platforms and rigs/setups (be that disastrously slow or lightning fast dependent on hardware) - this has benefited them no end. Apple on the other hand - to use Steve Jobs as a reference, has always concentrated on a much smaller number of products to do them right. Hence the tight integration of software and hardware, you buy a Mac because you want a well built machine that works for years without as many issues you would get on a PC, with alot of software you may well end up paying more for comparatively on a Windows based machine.
I myself am a big fan of the iPhone and iPad because as far as I'm concerned the experience you get on these as a user IS second to none (MY opinion before someone shoots me down).
Actual desktop wise, I'd pick PC EVERY SINGLE TIME because I build the units myself, I can put what I want in there when I want without having to check its Apple approved - and believe me if you've not built your own machine before - properly I mean not scouring to find the cheapest options (i.e. the crappy low power i5's etc...) the cost price difference is not that vast between Mac & PC - I'll happily spew my gaming rig setup to prove a point if it makes those non-believers happier, also Windows without all the bloatware crap that alot of manufacturers put on them is actually not too bad!
You will not on the most part change this same situation in the enterprise space, every corps requirements are different from the next, Apple simply does not cater for this market (currently) - hence their partnership with IBM giving them a much vaster reach into this market place - look at the bigger picture.
If you talk about notebooks, I'm indifferent, for work I use a 13" MacBook Pro Mid 2012 upgraded with 16Gb RAM as I use Parallels to run Windows on a Thunderbolt display. Regardless, if you look at what you can generally upgrade on a laptop - on the most part HDD/SSD and RAM are about it! - cost is more important for most here.
At home I use a Dell 15" i7 (dual core), 8Gb, 256Gb SSD, nVidia discrete GPU, as my home laptop, which due to my iPad I rarely need to use as I can remote into my gaming pc if I need something specific done and I can't be bothered to move to the chair.
I am an Apple fan, just also a realist who likes the best of both worlds without bias either way.