Triggering the thyristor for the single phase AC voltage controller in MATLAB
The issue is the gate threshold.

As per the thyristor block mask


  The thyristor moves into the on state if the gate-cathode voltage
  exceeds the gate trigger voltage. The thyristor moves back into the
  off state if the anode current falls below the holding current.


With a pulse amplitude of 6 and a default threshold of 6... the thyristor gating condition is never met.
If you set the pulse amplitude to 10 it will operate as expected.

One thing to note... there does seem to be a missing documentation tolerance here...

With a pulse of 6V I would expect EXACTLY what you are seeing... the SRC is not gated.
With a pulse of 6+eps I would expect the SRC to gate, however it doesn't.
With a pulse of 6+4.4409e-16 ( ie 2*eps)  the SRC is gating