There are several reasons for this. The order of mention of these reasons does not reflect the order of importance.
It is not unique to Japan, but like many other countries, there are found accommodations that suit whole spectrum of budget and purpose of visit. Backpackers for instance can find cheap rooms or dormitories offered by premises that cater for their budget.
Railway is a an efficient means of transportation with the city and between cities. The subways and the bullet trains provide excellent service. Information on rail transportation is readily available so that we can plan our journey or itinerary months before we leave our homes.
3. Tourist destinations
Attractive tourist destinations are in abundance, whether it is history based, Science based, entertainment based, flower/garden based or scenery based. Many well-kept gardens can become the main attraction for visitors. Looking at picture of Sakura and Wisteria bloom, can make one to fall in love with Japan.
4. The people
In public, the people of Japan show good manners and helpfulness. It makes our visit to Japan memorable.