Why do Dassault Falcons only have one thrust reverser?
As always, it is a trade-off.
On the positive side, thrust reversers allow you to slow down faster and reduce brake wear.
On the negative side, there are costs to install and maintain them. They add weight, which increases the required take-off length reduces the maximum payload and increases fuel consumption.
Dassault found the optimum for their design in one thrust reverser, others go with TR on all engines, none or optional.