It's because back then, if I remember correctly, new revisions of Macintoshes with PowerPC processors had CPU speed increments like, 80, 90, 100mhz, and meanwhile the Pentium was going 100, 200, 300, 400... (at least that's how it felt being a mac user...)
Also, I believe around that time the deal between IBM and Motorola was ending and they had already decided not to continue development of the PowerPC.
In the G4 era, the marketing back then from Apple talked about the "Mhz Myth" and largely showcased stuff that was pretty much "Hey, this hand-tuned AltiVec code blows away this C++ -> x87 code on intel!"
It's an interesting bit of history about how many very bright engineers had zero faith in intel being able pull off what they did with the P6 micro architecture: A superscalar, out-of-order x86 CPU that supported SMP on a power budget that, while high for the time (25 watts!), wasn't astronomical. That breakthrough ultimately became the foundation for Intel's Core micro architecture, which Apple later adopted in their first intel-based machines.
If Apple had switched to Intel with the arrival of the Pentium, things could have been very different.
Macs basically kept pace, pretty nose-to-nose, with the Mhz speed of Intel CPUs until the Pentium II era. Apple actually beat them to about 350mhz on the 604e for a year or so, whilst the fastest Pentium Pro of the time was about 200mhz.
This was quite a stretch of time (92->97?). What happened after that, though, was that Intel hit the afterburners, and rocketed up to about 800mhz, and PPC got left in the dust. There was a huge 500mhz speed bump that the G3 and G4 both stalled at for a few years, and by the time they got over that, the pentium 4 was coming out, which started another jump from the P3's 1.5ghz, pushing the P4 up to about 3.5ghz.
The G3 and G4 were much faster than either the 603 or P2/P3 and especially the P4, per mhz, so the launch day ad campaign saying the iMac's 233mhz CPU was "twice as fast" as the 250mhz pentiums that were out at the same time was true - it just didn't stay true once 500 mhz P2 machines came out less than a year later. It's notable that intel was so focused on the marketability of that metric that the later pentium series were worse per clock than their predecessors - a P4 had to actually be several hundred mhz faster than a P3 to be the same speed. Intel was primarily doing this to fight AMD, and it got bad enough that AMD actually had to introduce a new marketing term to indicate how fast their chips were compared to the mhz ratings of P4s.
Intel completely reversed course on this with the core series - modern "core" chips like the i5 and i7 are several times faster than P4s running at the same clock speed. Far better heat/power consumption specs, too.
Also, I believe around that time the deal between IBM and Motorola was ending and they had already decided not to continue development of the PowerPC.