How so? Duolingo is not some authoritative list of “languages that are important” where any language not included is, by definition, unimportant.

Sometimes my name isn’t included in those souvenirs that have lots of different names on them, yet it is not a mockery of me.

It probably should be included, but is it really a mockery if it’s not?

Duolingo has a platform where volunteers can contribute to develop new courses. If you are fluent both in Bengali and in English, you should definitely consider signing up.

https://incubator.duolingo.com

Yeah, that Duolingo is a for-profit company that relies on volunteers to develop their courses is probably a mockery of someone, but it also explains why nobody has gifted them a Bengali course yet. On the other hand, people who learn a conlang are self-selected for willingness to spend a lot of time on something that's unlikely to gain them any material reward, so it's unsurprising some of them worked on a Duolingo course.

There are also almost three times as many Wikipedia articles in Esperanto as in Bengali, despite the Esperanto Wikipedia having less than a third the number of active users: https://meta.wikimedia.org/wiki/List_of_Wikipedias/Table2

Somebody has to pay the costs of running the system.

The fact that these are available on Duolingo but not [INSERT MISSING LANGUAGE] is a mockery to [INSERT NATIONALITY] people.

More Duolingo users are interested in learning Esperanto than Bengali.

It comes down to demand and whether someone takes the time to create the lessons.

considering that there are about 300 million native Bengali speaking people in the world (mostly in Bangladesh, and West Bengal which is a state in India).

It's not how many people speak it. It's how many people want to learn it as a second language. How many people would be in a position to want to learn Bengali if they don't already live there?