The Zapp is a good folder though. Many foreign strollers are not so usable in Japan. With Japanese strollers, there are design differences - I wonder whether this is down to customer needs or if these Japanese designs have controlled how strollers are used here.

Babies aren't supposed to leave the house for 1 month (their first outing is supposed to be to their 1 month checkup. So strollers aren't designed for newborns. There are 2 types of stroller - A for 3-6 months and B for over 6 months you never see double strollers*. A's normally towards the parent or can face both ways and have a decent recline, but they are rarely fully flat and it's unheard of for a stroller to de designed for overnight sleeping.

B-type strollers don't need to be as big as US, as children stop using them earlier and are made to walk. So they can be designed to fold small and to fit train gates (max 55cm wide). Foreign imports are becoming more common in department stores, but are crazy money compared with importing them directly. They are usually too wide for train gates, so you have to use the disabled gates (85cm wide), there's always at east one of these , but you might have to ask to be let through.

In terms of trains: many stations (not older subway) have barrier free routes, but in old station they can be a bit odd (like asking to go the wrong way through gates and out of the station and reentering in another entrance for a transfer) staff will help and are required to carry up/down stairs if no elevator available. But if you don't know the stations it can be a pain and you need to allow lots of extra time for transfer if you go the stroller route. You start to get irritated by rude Japanese that use the elevator when they have no physical need leading to increased wait time - at busy stations with slow elevators, you might have to wait for several loads (5-10 minutes) before you can get in an elevator. Polite couples with strollers will often split up with one accompanying parent taking older children on escalators, reducing the number of people waiting for the elevator. On the trains, you used to be required to fold up strollers at all times, but now you can leave strollers open with children in. Obviously you aren't supposed to travel like this at peak times. On limited express/shinkansen trains you can put folded strollers behind the last row of seats. Some trains (like N'EX) have a wheelchair area at the front of some carriages that can be used for strollers too. Children under 5 travel free, but aren't supposed to use a seat if anyone is standing.

On other forms of public transport: there is space for wheelchairs on some buses too, but I can't say I've ever seen people with strollers on buses using these areas. Domestic flights don't normally allow stroller gate checking (I don't know how this works on the newer LCC though)- you check them as normal baggage and pick up from the reclaim belt (they usually come out first before the priority luggage). They always wrap them in plastic bags and I have not had any damage in many flights. When you check in, you can get an airport (or airline, these are often nicer) stroller, that you take through security and leave at the gate, you can ask for a stroller to be waiting at the arrival gate for you too.

Bottom line, strollers can be a pain if you have certain types, take longer to get places (especially if you are not familiar with the routes), so might not be the best choice for tourists. I use a stroller in Japan almost everyday, but we have several different strollers for different purposes and for many trips we leave the wheels at home. If you don't plan carefully, a stroller could cause many more stresses and problems on a visit to Japan than it saves.

In terms of popularity of baby carriers - nothing to do with development imo, just tradition (as in most countries before the stroller) and mostly laziness. Don't rely on Japanese TV documentaries for anything near science or truth. (Other factors are the high use of bicycles and public transport). However, I can't think of any families with young kids that don't have a stroller in regular use.

*ok, maybe not never seeing double strollers - I have one and I have seen another couple in my city with one a few times. But these are really really rare, for very good reasons.