@Antoine I would assume exhibitry reasons, look at the Hanuman langurs exhibit - it looks great because there is a massive group of them, 3-4 animals would easily get lost there. It would also require the "safety belt" to be cleared out around the exhibit so they don't jump off.
Some island could be an option as well, but all Ostrava's islands are occupied by lemurs (one of their key species as well) so they probably don't want to limit their space. Also, looking at the islands, one by one, it could be very hard to rebuild one in a way that the keeper can't get into direct contact with gibbons while entering the island.
Also, this way, the hind wall, and indoor exhibit actually cover some off-show area, so while only a cosmetic detail with no big importance, it could play part in the decision-making process too.
