This might not be the exact or specific answer you want but it should give you a relatively good idea as to the why. Most of the downloads were previously hosted on the site until it got to the point of people uploading absurdly sized zips because of poor habits in texture sizing (looking at you mr 8000x8000 solid image texture) which rapidly made the cost of the site skyrocket to the point of I believe were were informed by @liquidfire215 that it all needed to be moved offsite or they were considering raising the monthly payment to something in the 4 digit range or just outright removing the site. I don't think any of us really expected the site to get as big as it has, we don't sell anything or make any profit outside of donations or ad revenue and even the donations from what I can recall are quite rare since a large chunk of the user base is not of the working age range yet. I've thought about suggesting some type of store with basic things in the past but kind of figured it'd cost too much and not garner enough profit for it to be relevant. @Outlawed May be able to give you a better or more informative answer than myself.
There were other sites besides MediaFire but a lot of them no longer exist or would just delete your files or account the second they had too many people using the service, I personally always try to upload anything of mine to Google Drive in addition to MediaFire for those who don't wish to venture near MediaFire. The main reason for a lot of the sketchy ads they use is largely to do with their commonplace use of illegal activities. Google Drive will scan files for viruses provided they are below 100MB but if you've downloaded anything from Sim-Monsters recently you know some of the file sizes are rather ridiculous if we're being honest.
I highly recommend uBlock Origin and some sort of Pop Up Blocker if you are an active user of the internet.