SQL Server - Why does importing lots of columns with few rows takes way longer than few columns with lots of rows?
At first, I guess the size of the 268 columns can be pretty big. So just look at number of bytes in one row, multiply by 128 and you'll see how much data SQL can cache at one time.

Second, there might be a lot of data type conversions from text to numerics and dates.

At third, there might be foreign keys, indexes, constraints and even triggers on the table you are writing into.

At fourth, the table can be in another database, which is placed on a slower drive.

At fifth, I guess all 268 columns can't get into the same page, and records go to LOB pages.

So, there might be even more reasons. I might suggest you to use clustered Columnstore index on that table, that might improve the loading situation.