My vote is on that Creative intentionally decided not to support SB Pro in stereo. There is a good reason not to.
SB Pro stereo support is a mess. You have a mono DSP and then mixer is hacked to throw every other byte left and right. So you program the DSP to play 44KHz mono sound if you want 22KHz stereo. And to make things worse, the DSP is so weak that this is done in "high speed" mode. That means that the DSP cannot respond to commands since every bit of processing power is needed for playback.
It is a problem that stereo playback must be enabled from mixer. Some games forgot the mixer on stereo mode. When you play some SB 1.x or 2.x game after that, the game properly resets the DSP but doesn't even know that there is a mixer (since old Sound Blasters do not have one) so they cannot put the mixer to back mono mode. So what happens is that the mixer left on stereo mode halves the sampling rate. This is a problem with SB Pro and clones but does not affect SB16 because it does not support the mixer stereo hack.
SB16 has a proper programming interface: you program sampling rate (in sane format unlike SB and SB Pro) and then simply tell it to play 8 or 16 bit in mono or stereo. I'm sure Creative knew full well that there was no games that used stereo samples when SB16 was released. So it sure made things easier for them and it was in their best interest that SB 16 programming interface would be supported as soon as possible.
I'm sure they could have supported SB Pro if they wanted to. It would had required extra effort and even the mixer bug could have been fixed if DSP reset would have reset the stereo bit in mixer. But why would they have done so? It was completely unnecessary (and they sure had much more important fixing to do).