The current Ethernet 8P8C connector that carries 4 data pairs traces it's heritage to phone jacks carrying a varying number of phone lines. The first line starts in the middle, which allows using smaller plug in a larger socket. You think it shouldn't work? In modern offices, wired for Ethernet it's commonplace to re-purpose an 8P8C line for a phone or a fax and plug RJ11 directly into it. Whenever the RJ11 is wired for 1 (center) or 2 (center and one layer next) it works pretty well.
So the actual question is : why all 4 pairs are not wired center-out as a 4 phone lines would make most sense? If you look at RJ25, it carries 3 lines expectably, in a concentric fashion. Why Rj45 is strange?
The answer is that the original RJ45 was not in fact designed to carry 4 lines as the pincount suggests. It was designed to carry only 1 line for a modem and a little trick: a programming resistor. Value of this resistor would tell the connected modem roughly how long the line is so the modem would set the transmit power accordingly. But now, if they wired this resistor in place where a pair would be normally wired, then something bad could happen if a regular, phone multi-line plug got accidentally plugged there. So they picked pins 7&8, which do not constitute a pair under regular phone-wiring rules. So now even if you would plug a 6-pin RJ25, it would connect the central pair to the only pair, and one pin of the outmost pair to one pin of the resistor. Phew, at least no wrong circuit is completed this way.
And now Ethernet comes to the stage where most of the seats are already taken. So what do Ethernet designers do? They take the only pairs that remain guaranteed unused. The original, 10MB, Ethernet needs only 2 pairs. So it uses the second center line, the one not used for modem, and the other outer line, pins 1&2, not taken by programming resistor.
Long, long time later, the RJ45 phone wiring can be pronounced dead without a doubt. So the remaining "phone" pairs are finally free for the Ethernet to take, as Gigabit Ethernet dully takes advantage of.