it would require additional hardware to actually eject the film, which makes the craft heavier and creates more opportunities for technical conflict/ failure. ex. what if the film isn't applied properly and becomes "foggy", effectively making it a costly and ineffective addition? or the film catches on the craft itself causing issues with other systems? there is also concern about litter, although that is mostly a moral concern/ slippery slope argument as a few pieces of plastic on a planet is negligible. it could also be an ineffective solution if it doesn't remove the dust as well as intended. mars' sand isn't like earth sand; it's extremely fine and by nature more resistant to removal. there's a good chance the film could be ejected only to blow more sand on what it's protecting in the process if the aerodynamics is not accurately predicted. i'm not a scientist, but i'd guess static could play a big part here too. in any case, it would cost a lot of money, time, and research to make a functional film ejection system, which may piss off those financially invested in the project if it doesn't work. an issue like that could hinder future research. NASA is very careful about what they invest their time and money in, and has likely determined that the risk vs. benefit of a sand removal system is not in their favor. i don't think there's a good solution to this problem, but people tend to be creative and continuing a positive conversation about it may reveal one. :>