Because they don’t want to extend the lease themselves, this is often reflected in the price.
You also have to factor in the changes in the law that will remove marriage value that should be going through parliament next year.
Edit: Freehold not leasehold but still a few £k.
https://www.rightmove.co.uk/properties/139920194
Why would someone sell a share of freehold flat with only 98 years on the lease? Isn’t it easy to extend that to 999 and make it more attractive?
Also if you were the estate agent and it was only 2 flats in the building, wouldn’t that be a key selling point?