Why must software companies change the UI?

I like to think of myself as flexible and adaptive.  I'm pretty good at picking up new technology.

However, there are limits.

Last week I upgraded two of the software tools I use for work - Visual Studio 2012 and Telerik Test Studio 2013.

And I wondered who the designers had talked to before changing the UI?

It certainly wasn't me.  Or my coworkers.  Were any customers consulted?  Any customers over 25 years old?

The new flat, black and white UI is hard to read.  It's hard to identify the icons.  The font is smaller.  It looks like the UI from the Windows 3.1 days.  Wait.  Even that included some color.

And I wondered, Why?

Why change the UI at all?  Was there something fundamentally wrong with the old UI?  I didn't notice any problem.

Is it necessary to change the UI just to convince people to upgrade?

I know that many times the UI changes are driving by marketing concerns.

But, shouldn't usability be considered?