What drastically changed tourism in the 1900s?

Answer 1

mass production of the automobile and superhighways.

That's certainly one factor, but not the only one. The development of the railways in the 19C was the first major incentive to tourism; and still the prime mover for holidaymakers until at least half-way through the 20C, when mass ownership of cars and improvements in road transport generally took the lead.

Later of course came air travel at prices low enough for any reasonably affluent family to take holidays abroad, and with this, the consequent rapid development of the resorts - an effect the railways had brought.

Interestingly, in the UK the railways may be regaining some of that traffic as motoring becomes costlier and less pleasant; and the Channel Tunnel has taken a lot of traffic that would otherwise have had to use the conventional ferries.