Why NSWindow without styleMask:NSTitledWindowMask can not be keyWindow ?

Problem:
I have one window mainWindow and another childWindow added to mainWindow.
childWindow is kind of WindowExt class. This class I define for catch method call

[ NSWindow becomeKeyWindow]

that must be called after

[childWindow makeKeyWindow]

.
If I create childWindow and try to make it keyWindow on next way:

WindowExt *childWindow = [[WindowExt alloc] initWithContentRect:addedWindowRect
 styleMask:NSBorderlessWindowMask | 
 NSTitledWindowMask
 backing:NSBackingStoreBuffered
 defer:NO];
[mainWindow addChildWindow:childWindow ordered:NSWindowAbove];
[childWindow makeKeyWindow];

method

[WindowExt becomeKeyWindow]

for childWindow is called - all fine, childWidow become keyWindow.

But if I create childWindow as

WindowExt *childWindow = [[WindowExt alloc] initWithContentRect:addedWindowRect
 styleMask:NSBorderlessWindowMask | 
 backing:NSBackingStoreBuffered
 defer:NO];
[mainWindow addChildWindow:childWindow ordered:NSWindowAbove];
[childWindow makeKeyWindow];

without NSTitledWindowMask,

[WindowExt becomeKeyWindow]

for childWindow is never called - childWindow doesn't become keyWindow.