Explain in a few sentences why a historian might not think that engels’s description reflects a completely accurate picture of what life was like in england during 1844.

That is because Engels presented things much worse than they actually were so as to prove a point and spread his ideology. Of course that a little bias was necessary for that since it had to motivate people that they had it worse than they really had it.

Engels focused exclusively on the problems experienced by the working class is England, and did not give equal attention to benefits or advantages brought about by industrialization.  

Historians might not be so quick to say that Engels "presented things worse than they actually were," as the other respondent here suggested.  Engels relied on his own direct observations and had testimony from other eye-witnesses as well.  He carefully chronicled  the harsh and harmful conditions seen in the industrial town of Manchester in 1842 to 1844 -- years during which Engels stayed in Manchester and did his writing.  His detailed depictions of the ugly side of industrialization tend to be accepted by historians as accurate and consistent with the conditions that working class persons there faced.

However, historians might question whether Engels was presenting a whole picture of life in England in the 1840s.  While the conditions of the working class were dire, those factory workers' lives were not easy before urbanization and industry, when they eked out a living on farms.  And there was an overall advancing of the economy and standards of living that the Industrial Revolution was producing -- though at first the advantages went far more to business owners rather than to workers. 

British historian (and Marxist) Victor Kiernan, writing a forward to the Penguin Classics edition of Engels' Conditions of the Working Class in England, noted that there were other things going on in Manchester besides the ugly side-effects of industry.  There was considerable intellectual activity to be found there, for instance. "But it was a darker side of life that Engels's eyes were fixed on," Kiernan said.   

So a historian might point out that Engels presented a one-sided picture rather than looking at all facets of what life was like in England in 1844.  But that was Engels' intention.  He wanted to highlight the conditions of the working class, because that was not what others were paying attention to as they hailed the advances being made by the acceleration of industrial production.