MathJax only implements math-mode macros and environments, not text-mode ones, in general. Since the tabular environment is a text environment, it is not in MathJax's scope of operation. MathJax expects you to use HTML constructs to handle text formatting, so a table like this is better handled via an HTML <table> tag. That is especially true in your case, since the table doesn't contain any actual mathematics (you are misusing \(30-50\), as this means "30 minus 50" whereas you are using it where I suspect you mean "30 through 50". So this whole thing should just be an HTML table, not a table generated by MathJax.
But if you must use MathJax, you can use an array environment rather than tabular:
\begin{array}{|l|c|c|c|c|c|c|}
\text{count} & 30-50 & 50-70 & 70-90 & 90-110 & 110-130 & 130-150 \\
\text{Number of Employees} & 7 & 15 & 30 & 24 & 18 & 11 \\
which will probably get you what you are looking for.