'reason is understandable' is a correct and usable sentence in written English.
You can use it when you want to express that the cause or motivation for something is understandable. For example: "I understand why you made that decision - the reason is understandable."
The reason was understandable: MTV relied on what it called an agreement but never actually had a signed deal with the Osbournes for the first season, leaving the network open to bigger demands during the recent negotiations because of the show's unanticipated success.
That a musician might need to miss a rehearsal or performance for personal reasons is understandable.
The reasons were understandable.
The reasons are understandable, if not entirely admirable.
In subsequent seasons Contact! lost focus as older pieces and European composers came to dominate the mix, even if the reasons were understandable.
Yelena Kostyuchenko, a gay rights campaigner and journalist with the Russian newspaper Novaya Gazeta, told AP news agency: "The law absolutely does not define what gay propaganda is and the reasons are understandable because gay propaganda does not exist.
The reasons are understandable and predictable.
"It involves a pattern of actual and hypothetical recognition of reasons (some of which are moral reasons) that is understandable by some appropriate external observer.
The reason for their reluctance is understandable.
His commitment to preserving a sense of mystery is understandable; for reasons besides its name and size, Threesome Tollbooth calls to mind Norton Juster's beloved children's book "The Phantom Tollbooth," in which a young boy named Milo receives one in the mail and drives through it in his child-size electric car, finding entry into a fantastical kingdom.
This is understandable for reasons of cost, accessibility, and ease of genetic manipulation that facilitate mechanistic experimentation in a timely fashion.
