B+-tree is an optimization for slightly faster sequential read, when leafs are organized into a list.

LSM allow for very compact data representation because most levels of the tree do not change much through time, and this is what we are talking about here. This compact representation makes LSM trees faster at sequential reads too (less pointer chasing).

Also, you can differently construct LSM trees for different operations: higher LSM tree with more narrow levels allows for faster writes, flatter LSM tree with less but wider levels allow for faster reads.