Is R suitable for digital signal processing
Since the bulk of R’s DSP capability comes from the signal package which was ported over from the open source project Octave (itself influenced by MATLAB), there's no intrinsic limitation of R.

What you have picked up on, are ecosystem preferences. We learned MATLAB in university, picked up numpy/scipy/sklearn at work, so R isn't the first weapon of choice.

That said, there are some interesting "bridge" projects that you may be interested in exploring: https://stackoverflow.com/questions/5630441/how-do-rpy2-pyrserve-and-pyper-compare.  

There is also the excellent interactive python (iPython) project, now called Jupyter, that recently introducing support for R notebooks: https://www.continuum.io/blog/developer/jupyter-and-conda-r. So tools &amp; their inter-operability are converging!