The time was also right even 4-5 years ago when the last Arabic-language.SE beta was closed due to a lack of activity.
Arabic has more than 310 million native speakers and more than 270 L2 speakers world-wide according to Wikipedia.
And the language itself has a huge impact on many other languages like Urdu and many Arabic words are used in our daily life starting with A like algebra, admiral and alcohol and closing with Z like in zenith.
I personally don't care whether we could launch a new Arabic-language.SE or a Semitic-language.SE, but I certainly do agree that there are topics which have found followers, active users and the necessary commitment which are more restricted in their usefulness than Arabic or Semitic languages.
And from the perspective of an moderator on Islam.SE we could profit a lot from an active Arabic-language site. As a big amount of questions that arise on our site could at least partially be solved with the help of Arabic language experts.
Beside all of this Arabic is one of the six official languages of the united nations, while each of other the five other languages has at least one site for example:
and many languages with much less native speakers have a running beta see for example:
Ukranian 40 million estimated native speakers according to wikipedia.
Italian 90 million total speakers according to wikipedia.
Korean a bit more than 77 million speakers according wikipedia.
and even artificial languages could get a beta site launched.
I'm far away to say that these sites are less useful. In fact I think every language is a kind of reflection of its culture and I'm happy to say that I love learning languages and speak some of them more or less fluently.
Finally it is a shame that Arabic didn't get this chance and sounds a bit unfair that many proposals didn't get the chance to get launched.