I imagine lots of possible reasons.

Text-only format emphasises placing value on the news itself, not on attention-getting images.

Simple format plays well with RSS feeds.

Extreme plain design allows you to stealthily read it at work without it being obvious you aren’t working (less important today than it was in 2007)

Intentionally bucking modern design trends fits well with the demographic of the readership, who are tinkerers and nerd-types.

Keeping the design the same after all this time has become a matter of tradition and pride.

There’s no need to change something that isn’t broken.