Because reversers are not 'required'. They are available as a useful addition to retardation only, but not essential. No doubt the aeroplane was very well maintained. Most airliners are flying around with various items going unserviceable almost by each leg. It's a fact of life, and no big deal. A reverser U/S is just one of those things.
Until further information comes in, I suggest we don't go blaming this aspect. The important thing at the moment is to just establish exactly what happened with none of that daft speculation that took place with the Kenyan accident!