Ok, Alberto's response aside, I think that you are asking why you do not get an option to select 60Hz...
The output from the processor is a DisplayPort stream. A LSPcon chip is used to convert from DisplayPort to HDMI. This chip may only support HDMI 1.4 and thus 30Hz would be the highest supported refresh rate. In some cases, the firmware for this chip is upgradeable. You should check to see if Dell has a firmware upgrade available for it that adds HDMI 2.0 support.
Hope this helps,
I followed your steps, but it doesnt work.
The resolition option at 3840x2160 still offers max. 30Hz
When I try set it manually, it gives message: "the set resolutions is over maximum bandwidth capacity"
Probably Scott idea may lead to 60Hz, but I was not able to find the firmware upgrade.