Because the train company involved does not want people on passes to use their trains when they can sell those seats for more to regular customers.
As well as the high reservation cost there is also a small quota of availability for passholders.
You may well find that other routes involving France have high reservations. Thalys, Eurostar, International TGV routes, domestic TGV routes. These all share one thing: French state operator SNCF.
Unfortunately a handful of major operators are of the opinion that they should be able to take their cut of Interrail/Eurail passes while engaging in policies actively designed to prevent use by those ticket holders or extract significant extra fees from them.
Even more unfortunately Eurail B. V. are unwilling or unable to compel these operators to treat their customers fairly and are also unwilling to properly explain these issues to potential customers.
The result of this is the very poor reputation that Eurail/Interrail has gained over the recent past and the regular slating it gets on platforms such as tripadvisor.
You can travel with reservation free regional trains from Brussels to Lille and then take the domestic TGV between Lille and Paris for 10 EUR (limited number) or 20 EUR.