The extra distance for NY/NJ-Bristol cables to cover is "just" ~400-500km (extra ~7%), plus if you look at ocean floor map [1] it seems they prefer not to put the cable in the relatively shallow waters of Newfoundland or Nova Scotia (maybe this decreases possibility of a damage by anchors, or they compensate a bit in length by going deeper, i.e. shorter circumference due to smaller radius).
The other possibility is that the direct path passes underwater mountainous region with quite variable depths in mid-Atlantic [1], so going up-down or follow the valleys anyway would increase the cable length. The right answer is likely a combination of all these to minimize cable cost, maintenance and latency.
Ships anchors and trawlers are major risks for subsea cables. Deeper is thus better.
Underwater geography is also a major consideration. It's not just the going up and down, it's the risk of landslides, tectonic shifts, volcanoes, currents and icebergs scraping the ocean floor.
On top of that are geopolitics and enviromental concerns.
1901 telegraph submarine cables [1] were more straight for a similar route. Likely because it was essential to make them as short as possible going underwater, so most of them were going from Ireland to Newfoundland.
Facebook operates a vast global network connecting its datacenters and points of presence (POPs). When you connect to Facebook from, say, Tokyo your HTTPS connection is terminated in a local point of presence (likely in Tokyo or somewhere in Japan) and from there the connection is forwarded on to a datacenter in the US or Europe. This forwarding happens over Facebook's privately operated network backbone, which needs to be substantial enough to handle billions of users' traffic (not to mention internal replication traffic and so on). Renting capacity from commercial providers is expensive so Facebook and the other big tech companies are interested in building their own infrastructure for this purpose.
Of course, it would be possible to have a user in Tokyo connect to Facebook's network somewhere closer to the DC but a) getting onto FB's high quality, low congestion network ASAP will perform yield much better performance than leaving the connection running over the internet where it's subject to congestion and long SSL connection round trips, b) terminating user sessions in POPs allows much finer control over which datacenter users are ultimately sent to. Without this FB would have to rely on DNS changes to steer traffic and they are slow to propagate and crude.
Facebook is responsible for about 8% of all Internet traffic. They need subsea cables to connect their datacenters and to deliver traffic to end users.
The other possibility is that the direct path passes underwater mountainous region with quite variable depths in mid-Atlantic [1], so going up-down or follow the valleys anyway would increase the cable length. The right answer is likely a combination of all these to minimize cable cost, maintenance and latency.
[1] https://earth.google.com/web/@41.47661069,-40.80459324,-4414...