For this ELL question, a desire for concision motivated me originally to use 'somewhy' instead of 'for some reason'; afterwards, a user kindly advised that 'somewhy' obsolesced. Why?
Google introduced me to http://somewhy.com/, whose author delineates his confusion of the absence and his reasoning justifying 'somewhy' (identical to mine). Though his questions have not been answered and
" "Somewhy" didn't appear in the dictionaries I [he] checked "
,
OED's entry does not answer our questions either:
somewhy {adv.} = {rare.} For some reason or reasons.
Postscript: I read this Wikipedia page on pro-form, which displays the correlatives for 'why', but doesn't explain their obscurity. The lone prevalent exception is 'therefore'.