Simple answer. Registrars can charge what they want.
Want more? Ok.
Becoming a registrar requires a HUGE investment in network bandwidth, equipment, backups, personnel, fees, company website creation, and so forth. Because of the requirements both from ICAAN and the public at large, any new registrar cannot be a small effort. Any new .TLD such as .gofishing (just making one up), will require recovery of expenses as quickly as possible to recover as much as possible the initial investment. Why? Businesses must become profitable quickly. For registrars, large investments become yearly expenses such as renewals with ICANN and any equipment and network upgrades along with expansion of personnel. If the initial recovery of expenses comes too slowly, then the second year expenses can tip the balance sheet for the business into a non-recoverable scenario.
This means that established registrars have an advantage. While new registrars must scramble to recover initial investments within the first one or two years and not stretch too far into years three and four, established registrars have the luxury of having already paid back their initial investments and having built an infrastructure that only needs to be maintained. This allows an established registrar to lower prices attracting more customers while remaining highly profitable.
Also consider the market. For example, for .com, there is higher market value for this space. Why? because .com sites have more value and most people would rather have a .com site at least for their primary site. For .TLDs such as .gofishing, the market would be much smaller. This means that the registrar with such a strong and vast market for .com sites, can offer them at a lower price due to volume and demand and .gofishing with limited demand will not find profitability as easily if at all.