The reason is because there are restrictions on the forum to how many posts you can make a day and how many new threads you can create, which are all based on your in-game character's level. The posting restrictions and the levels they lessen can be found here. According to the posting restrictions thread, you still need another 4 levels or so before you can create a thread.

As for why SE has these restrictions in the first place, it was because back on the Beta test forum in 2013 when the game was still in development a large amount of new testers joined who were a little too zealous in their posting, spamming the Beta forum to hell and back with useless threads, and added even more stringent posting restrictions there to try and bring order to the chaos. Presumably hoping to avoid a repeat of that here on the official forum SE decided to implement similar restrictions here.

Unfortunately there isn't really much that can be done other than as you level in the game restrictions to how many posts you are allowed, how many threads you can create and where you can post here on the forum will gradually lessened or removed.