Frankly, it's pretty tricky to pull off good file encryption and decryption, even with a library that's supposed to do it for you. I tried to modify a cryptor (meant to obfuscate viruses to hide their signatures from antivirus programs) that used a very basic bitshift technique originally, and it didn't work well because most antivirus programs would literally brute force the binary and could realize that it was actually a virus! I wanted to replace the bitshift "encryption", if you could even call it that, with AES encryption via a C# library that I had used successfully in the past for strings, but I could never make it work. Another problem is that the more complicated your encryption algorith is, the longer it takes to sweep the entire disk, and then decrypt at the end when/if they pay. It's also more likely to fail in the middle and result in an incomplete job.
What I saw once on my grandma's computer, back before ransomware really took off and got better, was a ransomware program that supposedly encrypted her files and wanted about $200 to decrypt them. All it did in reality was add the extension ".crypted" to the end of every single file so windows didn't know what program to use to open any of them! Once I figured that out, all that was necessary to do was use the task manager to locate and delete the original ransomware file, then write a batch script to recursively check every file on the system for the .crypted extension and remove it if present. Problem solved within an hour, no money paid to hackers at all! But if they had used AES, this technique wouldn't have worked at all, and deleting the virus file would probably destroy any hope you ever had of cracking the military grade encryption.
The difference is that the guy who used the first technique of renaming the file extension so windows couldn't open any files probably already had hundreds of infections by the time the guy using military grade encryption even finished his virus, and the 5% of people who are smart enough to fix the first one are probably also smart enough to not be infected by the second one. The other 95% of people who couldn't figure out any way to fix either virus other than just paying the ransom, probably paid it the first time (to the guy with the easily breakable ransomware) and then immediately set up a full backup solution to prevent it from ever happening again. If they later got infected by the military grade ransomware, they already learned their lesson once and just restored from the backup. Hopefully even people who haven't been infected yet start to set up backups, so they never even have to pay the ransom once.
From this scenario, you can see how the guy with the easily breakable ransomware can release it first, it acts faster and is more reliable and easier to reverse even if the antivirus program manages to delete the virus before decryption, and because of that, he will make more money than the person who spends tons of time setting up a fully bulletproof ransomware virus, but is later to the market after people have started to wise up and back up their important files.