There are two answers to your question.
One is medical. It seems to me that some of the genes on chromosome 7 must be important for the proper functioning of bone marrow, so damage to those genes leads to the most serious problems with bone marrow.
The other is statistical. Different studies have given different prognostic ratings to abnormalities of chromosome 7 (monosomy 7 or chromosome deletions or translocations), but the results have shown that, in general, many possible problems with chromosome 7, especially in combination with multiple other abnormalities, are associated with shorter survival.
Not all damage to chromosome 7 produces the same prognosis. There are many details in an
NIH report from 2008, and no doubt in similar more recent studies.