Fortran has become popular and widespread because of its unique combination of properties. Its numerical and input/output facilities are almost unrivalled while those for logic and character handling are as good as most other languages. Fortran is simple enough that you do not need to be a computer specialist to become familiar with it fairly quickly, yet it has features, such as the independent compilation of program units, which allow it to be used on very large applications. Programs written in Fortran are also more portable than those in other major languages. The efficiency of compiled code also tends to be quite high because the language is straight-forward to compile and techniques for handling Fortran have reached a considerable degree of refinement. Finally, the ease with which existing procedures can be incorporated into new software makes it especially easy to develop new programs out of old ones.
It cannot be denied, however, that Fortran has more than its fair share of weaknesses and drawbacks. Many of these have existed in Fortran since it was first invented and ought to have been eliminated long ago: examples include the 6-character limit on symbolic names, the fixed statement layout, and the need to use statement labels.
Fortran also has rather liberal rules and an extensive system of default values: while this reduces programming effort it also makes it harder for the system to detect the programmer's mistakes. In many other programming languages, for example, the data type of every variable has to be declared in advance. Fortran does not insist on this but, in consequence, if you make a spelling mistake in a variable name the compiler is likely to use two variables when you only intended to use one. Such errors can be serious but are not always easy to detect.
Fortran also lacks various control and data structures which simplify programming languages with a more modern design. These limitations, and others, are all eliminated with the advent of Fortran90.