Why it's so cold this year?

If you mean for much of the central and eastern U.S. and Canada, the main reason is that there has been a rather persistent weather pattern this winter, with a ridge of high pressure generally along the west coast of North America and a strong upper low around the Hudson Bay area. The wind flow between these two features has been originating basically from the Arctic Ocean and northwestern Canada, and from there diving southeast across much of the central and eastern U.S. This has tapped multiple surges of very cold arctic air and pushed it southeast into those areas for much of this winter. Not an overly unusual pattern, just has hung on for some time.

I'll put it as simple as I can

There is a very intense ridge of high pressure by the north pole, and it's blocked by strong low pressure systems over Europe and Russia. The winds in a high pressure system flow north/nortwesterly which means frigid air is being funnelled in all the way down to the central portions of the United States.

Another big problem is the jet stream is running a lot further south then it normally would, which means again cold air sinks farther south then it normally would this time of year,

A third issue is the amount of snow that has fallen over most of Canada and a chunk of the USA, when there is a lot of snow on the ground it makes it harder to warm up the surface, and the snow sends cold air back up into the atmosphere, if that makes any sense.

I don't know where you are located, but anywhere from Alberta east to Newfoundland and everywhere from Montana east to New York and south to the Carolinas/Ohio Valley will be seeing below seasonal temperatures for the next 7-10 days, by mid March most areas will see temperatures much closer to seasonal. The weather pattern also has a series of clippers forming in Alberta then moving south through the Ohio Valley and then skirting up the east coast, if your on the eastern seaboard then expect to see several moderate storms coming up these next 2 weeks.

may be the due to the affects of global warming everywhere!

It's kind of related to solar cycle.