Why is the positive terminal of the DC source connected to the primary of the transformer?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) SW1 conducting. (b) SW2 conducting.

Figure 1 shows a simplified version of your inverter circuit with switches replacing the transistors.


When SW1 is closed current flows through the centre-tap through SW1 inducing a voltage on the secondary as shown by the arrow.
When SW1 is opened and SW2 is closed a similar current flows in the other half of the primary winding but note that the current direction in the coil has now been reversed. The induced voltage on the secondary is now also reversed.
If this cycle is repeated the phase reversal on the secondary will generate alternating current, AC, which is the purpose of your circuit.



  Why positive terminal of dc source is connected to primary of transformer?


Because the transistor switches are connected to negative and a positive connection is the only way to get current to flow.