There are many causes from web design to UX design, involved a transformed from simple static pages to dynamics, interactive apps, which required a holistic approach that focused on the user experience. This shift is a response of expansion of the internet as well as better understanding of user centered design. Mainly focus on usability, accessibility and satisfaction rate instead of merely into visual approach. The result will be designers revolve around user needs and behaviors to make sure the experience is functional and enjoyable.
More than that, rise of mobile devices like cellphones creates new challenges, as designers had to adapt to a smaller screen as well as touched based interfaces, the whole game of design altered. New tech and web data analytics allows designers to use as an insight to inform their design decisions, adding effectiveness into their works. With improvement to create more complexed interfaces, designers could focus on combining visual appeal into design but aiming more in user journey across different platforms. Thus, this could be another part how the transition from web design to User Experience Design, because the needs changed.