As we all know, 100G products is very expensive. Considering the cost, a lot of network builders choose 10G→40G→100G before QSFP28 come out. At that time there are 100G products such CFP, CFP2 and CFP4 ("C" for 100, and FP for "Form factor, Pluggable”). But the form fact of all these are huge even the newest CFP4 is still bigger than QSFP28. Please refer to the picture below:
After 100GBASEQSFP28 has been released, it provides a more economic way. We can choose 10G→25G→100G, because 100G Ethernet is formed by 4*25Gbps. QSFP28 form factor make it possible to install more transceivers in one 1RU switch.
With respect to 40GE, 25GE enjoys the cost advantages of single channel. If it is 40G, multiple physical channels are required, because it makes use of 10G technology. For 25G Ethernet, the key advantage is that many components have been completed: because 100G Ethernet is formed by 4 25Gbps. Therefore, the components generated using the two kinds of technology can be produced in mass, to drive down the price.
Besides, the consumption of QSFP28 is below 3.5W which is much lower than other 100G products (6~24W). The GEN2 100G QSFP28 released by Infiberone only cost 2.5W.