The main reason is that, because as this forum's tagline states, this is "a Buddhist discussion forum on the Dharma of the Mahayana and the Vajrayana.
Within that context, the Mahayana Buddhism forum is a good spot for general Dhamma discussion, because this essentially incorporates Vajrayana too... both have Buddhahood as their goal.
There are also the Lounge forums (here and at Dhamma Wheel) which by the nature, are less specific, and would be amenable to true pan-Buddhist discussion.
Live in concord, with mutual appreciation, without disputing, blending like milk and water, viewing each other with kindly eyesDhamma Wheel (Theravada forum)
* Here Comes Trouble