You used to be able to get them (occasionally) in Bury market ... but really the answer as to why they're not readily available to buy is the same as why any other domesticated, semi or quasi domesticated or artificially-reared eggs - other than hen, duck and - to some extent - quail - aren't, either. T'other birds don't, can't or haven't yet been, sufficiently 'changed' by selective breeding so as to lay eggs of a practically-useful size, day in, day out, year round and never give up ...
And if you fried an ostrich egg it'd be hard to give everyone a fair share of both yolk and white esp if they liked a runny yolk. You'd have to scramble it!