Because you can ride it on your car licence, that is if you got your car licence before 2013 and without the helmet (not recommended). Perhaps you have a partner that perceives three wheels safer than 2. Health issues such as poor balance, bad back etc. are all reasons trikes are chosen.
Although there are different types of trike such as touring trikes sports trikes by far the most common are cruiser style trikes.
There are many reasons why people ride trikes one of those reasons is because the rider has some sort of a disability perhaps has had a stroke or a balance problem or something of that nature. A few trikes have automatic transmission left-hand controls or something of that nature to make it more rideable. The TrikeandSidecarExchange can advise on this.
Trikes made from touring motorcycles Pan Europeans, Goldwings, Harleys that sort of thing.
By far the most popular and these are made from bikes similar to the Suzuki Intruder 1400.
Typically Honda Blackbird and Suzuki GSXR1100s are used.
It often surprises me how much more power a trike needs. So typically a 500cc twin-cylinder trike is really only for short distance although there is always someone who will ride long distance and says it's no problem.
Trikes like sidecars tend to wobble the steering at about 21mph. You can reduce this or stop it by doing various things. The most expensive fix is to fit Unitlink or similar leading link front suspension but at about £1500 it is an expensive but good solution. Steering dampers help and square section front tyres help but dampers and square section tyres then makes the steering heavier. My view is just put up with the wobble? There is nothing wrong (in most cases).
Trikes made by trike building companies are generally better because they have usually had lots of practice at it. Home built trikes are often a work in progress however some are excellent.
These typically come in 3 choices..
One or two-wheel drive. Chain or belt but most common is the drive shaft with a solid back axle, this is quite a good compromise between cost and performance.
Narrow trikes steer better. It can cost more to make but it is probably worth the extra expense, it will also fit in the back of a van and generally be a better ride.
Get it delivered. Riding home could be stressful if you are new to trike riding. Delivery is expensive because unlike having a solo bike delivered by a courier that has other paid for deliveries in the same van the delivery for a trike could well be a dedicated journey… but try to get that included in the buying price if buying from the TrikeandSidecarExchange or another trike dealer.