It just occured to me... There are trioses, tetroses, pentoses, hexoses and heptoses. Of these pentoses and hexoses are most popular and abundant. They have one thing in common - they can create a reasonably stable rings that are used in the polymeric compounds (polyhydrocarbons and nucleic acids - quite usefull in all living organisms). If these polymers are more stable than other similar compounds that can be the reason for pentoses and hexoses abundance.
I wonder how much starch and celluose stability can be atributed to the six atoms ring they are made off. Are there analogous compounds made of heptoses? Cycloheptoses? Tetroses/cyclotetroses?