The reason why is really the intersection of 3 factors.
1: NXP designed this way.
2: Many difficult decisions had to be made about which of the chip's GPIO pins to bring to the outside edges of Teensy 4.0 and 4.1. It was discussed for many months on this forum in the lead up to Teensy 4.0 and during a lengthy beta test period. Ultimately the decision was made to bring out 3 of the 4 I2C ports. Decisions that maximized many other features people wanted just happen to have selected the I2C pins which also have analog capability.
3: Since Teensy 3.0 we've tried to follow certain Arduino pin conventions which go back to the earliest days of Arduino Diecimila, Duemilanove, and Uno, such as Serial1 on pins 0-1, SPI on pins 10-13, analog starting with digital pin 14, and I2C on pins 18-19. So I2C1 and I2C2 ending up on analog pins was pretty much by factors #1 & #2, but I2C0 on pins 18-19 (A4-A5) was an explicit design goal.
These same factors will (probably) apply to future Teensy using other chips.