I have a Honda Trigger motorcycle. I have seen most of the motorcycles even like Karizma ZMR having 5 gears.
What would be the reason to have just 5 gears even for 223 cc motorcycles?
There are three considerations when making a low end/small capacity engine
2 and 3 directly affect 1.
To make a gearbox more reliable you need to make the parts more durable, this is normally achieved by using more materials or more robust materials which add to the cost.
To make a 6 speed gearbox fit in the same space as the 5 speed engine you need to make the gears smaller (not in teeth count necessarily but probably in width). To maintain the same reliability/durability you have to use more expensive materials again or increase the space used which means that the bike weighs more/takes up more space and may not be suitable for the market it was originally thought up for.
Additionally a small capacity engine may not ever reach the road speeds where a 6th gear is appropriate and may actually cause worse efficiency (i.e. if the engine is lugging down due to the low revs) or you would have to shift gears more (where the 6th gear ratio is equal to the original 5th gear and all other ratios are that bit closer to each other).