There are lots of tanks in game that do not have their maximum possible ammunition. Leclerc is no exception to that.
Both only just added to the game in the last 3 patches.
Most nations are using even older than that.
2A6 is one tank. Leclerc was already a powerful tank before, but was let down mostly by the fact it had no backup. Now we have a second that situation has improved and there is no need at all for the S.2 to have better ammo right now when the S.1 by itself was already strong and the ammo is still one of the best shells in game.
If the situation calls for new ammo, it will be considered. But right now, there is no need at all.
Lots of top tanks don't have their maximum possible ammo.
Leclerc was already on par with most nations tanks and was actually ahead of a few too.
There is no need for this shell at all at the moment.
Its not purely about year of introduction.
Leclerc is on top of many MBTs.
2A6 is being added alongside the T-90. Consider in quite a few cases, Leclerc won't even fight 2A6 but alongside it. Even when it does, it's current ammo is more than sufficient.
F.2 is a leap ahead that we don't need right now and isn't coming at this moment. S.2 will be strong enough.