that's because the plastic injection molds used to makes these frames can cost upwards of $100,000 to make. it's simply not feasible to make a mold for a custom tank frame that you do not sell a lot of. you would have to sell a lot of frames to even cover the cost of the mold
the reason why you see 55G frames around is because it's a standard size that a lot of people have, thus, you will sell a lot more frames for that size.
the only 2 places online that i have found that sell these frames are
http://www.aquariumframeswholesale.com/
http://www.glasscages.com/
This place used to sell them but they are no longer in business
http://cciaquarium.com/