Legitimate question: what do they hope to achieve by this?
Historically, the people here have engaged in "knee-jerk" violence following an initial inflammatory incident; see:
https://en.wikipedia.org/wiki/Black_July
https://en.wikipedia.org/wiki/2018_anti-Muslim_riots_in_Sri_...
In the second incident listed above, social media was used both as a vector to spread hate speech and misinformation, and also to help mobs organize.
Regarding the current social media block: at the start, I felt this to be reasonable, as it made sense to slow the spread of misinformation/hate speech for a couple of days, until people's emotions cool down. However, the blocks still continue - and I see no clear justification for continuing them for so long.
Shutting down communication channels rarely helps defuse such situations, though. It leads to more isolation, more rumors, more enmity.
shakes Magic 8 ball
My guess is it's a combination of what the Sri Lankan poster above mentioned combined with a reaction to further technical escalation by the citizenry.
Unfortunately, there are too many unknowns w.r.t. Sri Lanka's government/cultural values for me to even hazard a guess as to what the endgame is beyond minimizing net driven attempts to organize.
What I do see potentially transpiring, however, is someone figuring out how to host basic net based communication within the country; odds are though, the government feel themselves sufficiently equipped to take down in country rabble rousing, and possibly aim to keep outside influences from inflaming things further.
Best thing you can do if you want to get out, is find the IP addresses for your particular of site, and use that. DNS based blocking does squat if you're maintaining your own registry of IP->Domain Name bindings. If they catch on and start trying to IP block, well, welcome back to the pre/early internet days. Hopefully you know someone willing to host you a forwarding proxy.
Stay safe Sri Lanka folks.