The idea was to avoid conflicting route names with earlier games. Johto and Kanto were right next to each other so Gold/Silver just continued the route numbers where R/B/Y left off at #29.
Hoenn did not join on to Kanto/Johto so they started the routes from scratch, but instead of starting from 1 they used 101, 102, etc. So 115 is basically route 15 in Hoenn. The same is true for Sinnoh, they just started from 201.
It makes it easier to specify a route - if you say "Route 1" you need to specify "Route 1 in Sinnoh" or "Route 1 in Hoenn" etc. I guess you can think of it like a zipcode, where 1 at the front means Hoenn, 2 means Sinnoh, and 0 (as in 001 or 029) means Kanto/Johto.
However, the idea in Black/White was that Unova is very far away from the other areas (because it's based on America instead of Japan) and it was sort of a "reboot" of the franchise, so they decided to start from 1 again.