It is GSM-standard.

If it were 468, you would have asked the same.

Because an SMS can use either a 7-bit, 8-bit or 16-bit alphabet,
the number of bytes needs to be even and a factor of 7 (to
avoid wasted space).

To achieve the elegance of having a number that's a multiple of
10, the choices get further limited to 70, 140, 210, 280, etc
bytes.  I can only assume the choice of 140 bytes (160 7-bit
characters) was made over 210 bytes (240 7-bit characters)
because it seemed to be the best compromise between usefulness
and available resources.

John

Because that's the most they could get into a single GSM signaling "frame"
(I think that's the right term).

Yes, but you can send 3 x 160 in one message (network dependant)