There were (are?) patent issues with mp3s, so to actually have Reason handle them internally might require licensing fees for every copy, and I honestly don't think it's worth it to pay for that when we have a million easy ways to convert audio files.
It might be alright though to allow export to external compressors like LAME or FLAC, but to make it very easy to use they'd have to code specific interfaces for each program, and again, it's probably not really worth it.
Do people really want to export their songs from Reason *only* in a format using lossy compression? If not, why not just export uncompressed as it is and use a simple conversion program? There are some that are so simple they just sit there and wait for you to drag a file onto them, then they quickly convert it to the format you've already set and it's done.
I wouldn't recommend mp3 import of any kind though. You can convert and import if you want, but if someone gives you an mp3 and wants you to use it to *start* working on a track, even if you have to tell them 200 times it's not acceptable you should do it until they give you a reasonable version.
I guess that's another possible downside to mp3 output directly from Reason. It might encourage people to just export mp3s and leave anyone further down the line who has to work with the song stuck trying to explain what an uncompressed 24 bit audio file is and why they need it instead of an mp3...