Why candidates don't join usually even after accepting the job offer ?

1.Current employer may counteroffer in an effort not to loose you

2.Candidate receives two additional offers with additional benefits to include medical, dental, etc.

3.Candidate wants to see their current market value as it relates to their current position and/or salary

4.Any personal reasons(Travel,environments,work hours,etc..) 

Rule of thumb- Always keeping your options open with 2-3 offers on the table. Weigh all your options and take the best offer possible including salary, benefits, and additional perks. Never burn bridges with your current employer regardless of your situation being bad, good, and/or in different.