can someone tell why UTP 10/100 is wired 1236? why not 1234, 3456, 5678? (i know that on old adapters only 1236 pins were connected) which was the rationale of 1236?
EIA/TIA-568 defined the pairs 1-2, 3-6, 4-5, and 7-8 long before twisted-pair Ethernet came up in the form of StarLAN which was designed to utilize the already existing cable plants.
Possibly, StarLAN developers left the 4-5 pair out to simplify coexistence with the previous twisted-pair application POTS. Unless there's a good source that is guesswork and off-topic here.
10BASE-T and 100BASE-TX inherited that pinout. 1000BASE-T onwards uses all pairs, not just 1-2 and 3-6.