A few things:

• In France, a curved croissant indicates the use of margarine whereas straight shaped ones used only butter for the fat content.
• British artisan baking became a thing over the past 15 years or so, originally with most bakeries running with French head bakers so the above tradition made the jump over here. Nowadays most small bakeries run with a majority of British staff, however the look of an artisanal croissant has become established in everyone's minds. Supermarkets and other mass producers have then imitated the independents.
• It's more practical for both the consumer and the producer. The ends don't fall off, they tesselate better (particularly when freezing down unproved croissants) and are easier to shape.

Personally I prefer the look of a curved croissant but it's been 10 years since I worked anywhere that shaped them that way and even they swapped over while I was there to save a bit of time. It's just more effort for no real gain in an industry where margins are paper thin.