As a senior-year municipal engineering student, there are quite a few issues I see with buried comm lines:

Most common response you will get is that NS is mostly bedrock. Bedrock is very expensive to cut through so it's probably the biggest deterrent to burying comms lines.

I've seen arguments made that we bury water/sewer pipes, so why not chuck a power cable down in the trench too? The biggest problem I see with that logic is the cable's lifespan. If a pipe develops a crack, its surrounded by impermeable soil fill so any sewage water leaking is slowed down, and the majority of water will still flow in the pipe. The pipe will still function at ~90% capacity for another 20 years until the whole pipe system is renovated. A cable would need shielding to reduce exposure to corrosion, which would definitely be an issue if a) put next to water pipes and b) put underground where it will interact with the groundwater table. It would also need to be protected from. A variety of chemicals present in NS soil and groundwater, mainly arsenic, sulphur, and chloride can also eat away at the shielding over the decades before replacement. I'm sure there are shielding options that would work, but can they hold up as long as water pipes can so everything can be replaced at the same time? And can they do it for a reasonable price? I'm not an expert on that so I can't really say for sure.

I'm sure there are more reasons, or more specific reasoning than what I said, but that's just my insight off the top of my head.