quite simply, because the goal is no longer to produce able engineers with useful tools, but instead to produce consumers that will buy disposable products of minimum acceptable quality.
the 15c was a quality product that would last a lifetime. the person who could master it would attain a level of skill that would see them do well in their profession. but HP (and many other previously 'quality' companies) discovered that this was not the way to maximize shareholder revenue.
instead we provide students with tools that are designed to become obsolete, to break, and whose purpose is as limited as the market will withstand.
it is not just an HP problem, it is becoming almost impossible to buy ANY quality scientific calculator that is not geared exclusively to to what has become a dumbed-down educational system. today's calculators are not designed as general purpose problem solving tools, but as "answering machines" where the student can type in (verbatim) a limited range of questions, and have the machine magically produce an answer that student can write into exam paper and teacher can with little effort assign a grade to.
but we are all happy about this, as it maximises shareholder revenue - revenue with which we can all buy pink plastic flamingos and fruit-flavoured vodka mixers.