a transistor can work either as a switch or an amplifier and having different load lines of AC and DC can be explained, to my opinion, of its use as a switch where it is likely to be operated by the DC where we can't use AC which is regarded as High power on a transistor since it does'nt stand that, and the AC to be the feeding of the load the transistor used a control switch to which needs the thyristor to pass through before it reaches, the AC, the load. I hope that sounds logical!
For two reasons. . .?
1. because transistors need biasing; the b-e drop is 0.7 vdc to 1.0 vdc while the AC signal is at almost any level. If transistors turned on at zero volts the biasing circuitry wouldn't be needed.
2. a circuit can be AC coupled or DC coupled.