Why don you rewrite GnuCash in programming language xyz so that I can contribute easily?
The quick answer is “We won’t”. The longer answer is complex but still amounts to “We won’t”. GnuCash is a large body of code maintained by a small group of developers who are comfortable in C and Scheme (Guile). Actually, 80% of it is in C and approx. 13% is in Scheme/Lisp. There is no valid reason that would justify rewriting this amount of existing code in a newer language.