Some suggestions -
(a) wingtip treatment is an alternative, but generally not as good, way of getting what you get with a longer wingspan.
(b) we have a massive investment, worldwide, in airport infrastructure
(c) for the recent larger aircraft, wingtip treatment is a far cheaper alternative to not be able to get into existing aerobridges
(d) for smaller aircraft .. marketing .. they look good/modern/pretty/technologically with it/etc ... pick your choice
(e) as an add on there are structural penalties especially in fatigue but that may be a far better option than a very expensive wing redesign ?
(f) and, as Stallie suggests, not really a proposition for short range ops