The part of sentence "underlying reason why" is correct and usable in written English.
This phrase can be used when one wants to describe a reason for something that is not explicitly stated, or is not obvious from the surface. For example, "Many believe there is an underlying reason why the government is pushing for reform, but no one can agree on what it is."
The immediate, if not necessarily underlying, reason why a majority of Jeremy Corbyn's fellow Labour parliamentarians have lost confidence in his leadership is his performance during the EU referendum campaign.
Now soft money has become just another part of the American political landscape.The underlying reason why Americans spent so much on politics in 2000 was in part because they had so much money to spend.
There is an underlying reason why Renaissance echoes may haunt this image: the Renaissance haunts all images.
However, the underlying reason why these methods fail is that they do nothing to promote greater efficiency.
Which raises an awkward but key question: are these high numbers a pre-World Cup bonus or the underlying reason why England are wrestling for consistency?
But the main underlying reason why many journalists have a negative view of him is that he hasn't always been straight with them.
Little has changed.There are underlying reasons why the south is more troublesome than the east: its tribal structures are weaker, making it harder for elders to make deals stick; it is more remote from Kabul and the main trade routes; the population is less educated and more xenophobic; and it is the ideological heartland of the Taliban.
We did not explore the underlying reasons why disparity existed between some questions within distractor categories.
"If Sterling does not want to stay at Liverpool, fair enough, there may be underlying reasons why he does not want to stay.
Future research should explore the underlying reasons why older men may perceive problems with the availability of services.
Future research could employ process analysis to further explore the underlying reasons why inappropriate bed use occurs and to test the effectiveness of process improvement techniques in reducing inappropriate bed use in acute care.
