1) PoDL is substantially lighter, smaller, and simpler (though not necessarily cheaper) at 1 Gb than at 100 Mb, due to the increased frequency separation. Just like with other Ethernet protocols, the lowest frequency of comms is basically DC; it's only statistically brought above that by the scrambler, but there's no useful true lower bound. Having an order of magnitude more separation, such as it is, allows a more reasonably sized filter to stomp over less (ideally, approximately none) of the data.

2) Only the 1 Gb protocol includes FEC, 100 Mb is a simpler, non-error-correcting encoding. This means that even though the maximum frequency on the twisted pair goes up by an order of magnitude to ~660 MHz, requiring better cabling, better twist spacing, etc... it allows a "sloppier" job at both high and low frequencies, since the FEC really does hide a few errors. This can be spent on even worse filters for PoDL, on frequency-specific interference (e.g. an RF amp running nearby), etc.

Basically, I was surprised to find that 1 Gb was not only not more challenging at the system design level, it was often simpler. (I haven't played with 10 Mb in either of its two incarnations seriously yet.)