Why is Cat5/Cat5e Ethernet cable rated 100MHz?
If you start at https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair, and do some more googling, you can find this stuff out. 

But start about 1991, when the first version of TIA/EIA-568 came out. This is the standard which establishes cable categories for voice/data cables for wiring buildings. At this time, 10 MHz over UTP (100Base-T)had been around for a couple of years, and the standard established 3 categories of cable for signal/data use.

Category 1 - voice/analog only, up to 1 MHz. Although it was not specified for data, it was often used at low data rates.

Category 2 - Data to 4 MHz.

Category 3 - 10BaseT, data rates of 10 Mbit/sec.

Of course, things were moving quickly, and 100BaseT (and others) soon came out. This caused the next version to be released, with

Category 4 - 20 MHz, for a standard which never really became all that popular.

Category 5 - 100BaseT, which became the standard, and which could (especially at Cat 5e) also support 1000BaseT, or Gigabit Ethernet.

Due to the way data is encoded, the data rate is actually somewhat higher than analog bandwidth, which is why Cat 5 works at gigabit rates.

And finally, in the latest release,

Category 6 - 10GBaseT, or 10 Gbit Ethernet.