Most likely because Sasuke didn’t have his Sharingan unlocked yet. Until that night.

We seen Obito/kakashi jump straight from a 2 tomoe to a MS instantly when rin died. If Sasuke had awakened his sharingan beforehand it’s possible he would’ve awakened it at that moment.

@Tim Xman Makes sense, but I think that once he saw his parents dead at Itachi's feet, it'd be enough to awaken his Sharingan, so Itachi's genjutsu should've awakened Sasuke's Mangekyo in my opinion.

@Httpedrokkk it's probably because he was still processing what happened and as he had to escape he didn't really have the time to think what he just withnessed

It kinda all registered as one event I assume

He awakened Sharingan after the whole thing, and all the other stuff was just reminding him of it rather than new trauma

Basically what everyone else said, Sasuke didn't even have his Sharingan yet and one of the prerequisites for unlocking MS is having the base Sharingan in the first place. Honestly I think the reason some people jump to two-tomoe when the first unlock it is due to latent potential, but that's just headcanon.