In addition to the other reasons people have listed,
Microsoft desperately want to get you used to the
idea that the whole thing is a service that you subscribe
to and that they provide. It's not your PC anymore. It's
their services device. Some people will like the new
approach, but the Microsofties know perfectly well that
for most people it will be a hard sell, just as Metro has
been a hard sell.

People are getting invitation popups.
Why? Because they've already allowed Microsoft to
have unrestricted access to their machine and MS
is pushing new product.

If you sign
up then MS gets your email address and personal info.
You become one of their spied-on services customers.
But they're also trying to set the tone: You sign up
when the popup ad tells you to and you don't have
to think about it anymore. MS will transform you computer
as soon as Win10 is ready, as long as they have your
permission. Once that's done, it's in their hands from
then on. They'll let you know if there's anything you
need to know.... which will probably be mostly "special
offers".
You won't even be able to choose about whether to
have updates installed once you've got onto the Win10
services bandwagon. It's all a very gradual
transition from selling to renting and from software to
services, which Microsoft has actually been trying to
pull off ever since they came out with Active Desktop
in 1998 and "invited" you to subscribe to ads on your
Desktop. Their quest became more urgent when they
saw Apple making over a billion dollars a year through
iTunes alone -- selling crippled music to a captive
audience of suckers. Services is all about a captive
audience of suckers. But for the general scam to succeed
they need to get people used to the idea that while you
might pay for a car, you're actually just going to get taxi
service. Ideally they need to make you into an AppleSeed:
someone who thanks them for exploiting you.