Q: Why isn't Mathematica not as popular as MATLAB or Python? I found it much better on visualisation but not many people using it. Why isn't mathematica not as popular as matlab or python?

A: MATLAB and Mathematica are different packages for different user groups.  Both software tools have similar freeware versions including Octave (MATLAB), Maxima (Mathematica), and others.
MATLAB has realized its objectives as being a software package for prototyping, idea testing, and simulations.  It is more popular with engineers and some scientists.  This popularity is based, in part, on marketing to students and instructors, ease of use, and availability.
Mathematica is a difficult and yet powerful tool.  It has a set of language options and the usage has been personally frustrating.  I was excited about integration with R language, but I have found that integration to be lacking, so far.  It is more popular with a few engineers, some scientists, and mathematicians.  Mathematica's latest editions have desirable licensing for students and home users.  While not free, expense is reasonable.
Both tools have limited use in some industries. 
Licensing is much more expensive for industrial partners.  Industry continues to tighten its belt.  Therefore, if you plan to be in industry, it may be highly beneficial to experience the freeware options, which nowadays includes the packages mentioned above as well as R, Python, and others.
