There are many reasons why they leak in wav, and you're really asking several questions in one.
All songs originate from a source, be it a DAT, CD, tape or vinyl. Some songs sourced by collectors were received as digital data files, and some of those were actually in WAV format to begin with. Other times, collectors would get their hands on personal cassette tapes Tupac brought from the studio to bump in the car, or to give to friends and family. Even though the tape format has its limitations in quality, a raw WAV rip will still contain more information about the sound than a compressed file. If you wanted to remaster or mix any selection of these songs for your own mixtape (which a lot of collectors have done), a WAV file will be a better work print than an encoded file. Some songs came from a bad quality source, but encoding those songs to MP3 would just take even more away from the quality, and would limit the sound spectrum if you wanted to do any kind of signal processing.
I believe many of these leaks were ripped straight to WAV from the source material, especially the SIR and AEOM original albums. The pre-Death Row leaks were from a tape source, but even those were probably ripped to WAV from said tape.
With today's harddrives, it makes perfect sense to leak in WAV. You will never be able to buy these songs, and a while after these leaks are done they will start to get re-encoded, YouTube'd, remixed, and you won't find them as easily all in one place anymore. If you want to burn a CD of unreleased songs in a year from now, having the WAV files means you will have the best possible quality of the songs. You can burn them as an audio CD to store on your shelf and rip MP3's from that CD and you would have top quality MP3's with little to no quality loss from the actual burning of the CD, so you don't even have to store the WAV files. If you choose to, hard disk drives these days come with 2 TB+. WAV rips = make your own true 16bit 44.1khz audio CDs without quality loss. MP3s = will never sound as great on any system greater than a boombox.
no