They (the CB250) weren't huge sellers in the US - by this time (early-mid 70's), the US market was for bigger machines, and particularly the SOHC Fours. Even the venerable 450 and 350 had started to fade from the picture by then.
Honda made several attempts to maintain the smaller bike impetus, but to no avail.
The 175's were always good sellers for them, and they tried a disastrous replacement with the CB200, which never took off in the US; they're unusual (but not rare) over here.
Americans traditionally always thought BIGGER, though that is changing now as fuel prices skyrocket out of control.
And that was the situation with the CB250 - why get that when I can have a 350 or 450 for not much more money.
bill