One problem with a Segway is that there isn't great space for it. It's far too fast for sidewalks at 10MPH and it's too slow for bike lanes where cyclists often want to go 15MPH. We don't have space to accommodate them and cities are often unwilling to make even minor changes to their infrastructure, never mind the large changes that would be needed to accommodate Segways throughout cities - and take away space from other modes of transit like cars.
E-bikes piggyback on roads and bike infrastructure that cyclists have been using for a long time. It's often less than ideal infrastructure, but it works. Likewise, bike parking accommodates e-bikes in a way that it might not accommodate a Segway as well.
E-bikes are also a lot cheaper. $1,000 can get you a solid e-bike (RadPower just launched its RadMission 1 today at $999). While many e-bikes are $2,000-$3,000, that's still around a third of the price of a Segway. Walmart has some e-bikes for around $700 which are nothing amazing, but have more speed and range than a Segway.
E-bikes typically have a lot more range than a Segway. While a Segway X2 SE has a range of 12 miles, e-bikes typically start at 25 miles of range for low-end bikes on very high assist levels. My e-bike ($1,500 purchase price) is rated for 50 miles and I can usually get 80+ on eco mode which still offers decent assist.
I think there's definitely utility in mid-speed (10-30 MPH, faster than walking, not quite car speed), battery-powered, lightweight transit devices. I just think that Segway missed the mark with something that was way too expensive, put riders in a riding position they didn't really like, had a control system that didn't seem to connect with people as much as it needed to, and didn't fit in with existing infrastructure enough. We're seeing a huge surge in demand for these type of devices, just not Segways. Heck, Ninebot (who purchased Segway) is selling loads of electric powered razor-style scooters.
I loved the iBOT. When I heard that the creator of the iBOT was making a mobility device, I expected something amazing. The iBOT was a wheelchair that could go up and down stairs and keep someone at a 5-7 foot height. It seemed amazing. Then the Segway was introduced and I thought "how on earth could someone think cities would be reconfigured for this?" It seemed to ignore what we knew about what people liked to ride while being incredibly expensive and generally less useful.
I can use my e-bike for so many things. I can get around my city with little worry about range and I'm usually faster than car traffic. I don't have to worry about range because I have so much extra. If I had a Segway, I'd be moving about at under 10MPH (with traffic signals and such) and I'd be constantly worried about range. 12 miles is a short distance for a round trip. If I travel 6 miles on my e-bike, I have at least 40 miles of range left. If I travel 6 miles on a Segway, I'm down to half my battery - or less if there were hills. I might not have power to get back.
The Segway was just never useful enough and it cost so much. E-bikes, electric kick-scooters, electric long-boards, etc. are all showing that there's a big market for mid-speed mobility tech. There just wasn't a market for Segways.