I'd say this is because the game mechanics that come with ice locations are best left to the late-game -- or at least, the developers see it this way. Bearing in mind the young demographic playing Pokemon games, it is likely in their interest to leave the game's most complex overworld and battle mechanics to later in the game.
This might be explained best with some examples:
Many ice areas involve hail weather. This obscures the overworld, and introduces a battle mechanic young, new players may not grasp immediately. Introducing this later means it isn't lumped together with all the other technicalities of RPG games that young players will already be confronted with.
Some of the locations you mentioned are ice slide puzzles, which are tricky to navigate -- especially for young players. I remember Ice Path frustrated me as a kid: there'd be plenty willing to give up if it was at the start of the game, and weren't already used to how the overworld works in general.
Plenty of the ice areas in Pokemon are slow-moving and difficult to navigate: examples include Sinnoh Route 217 and Kalos Route 17. They're not a lot of fun, and the developers are probably aware. I believe these exist to add variety, which would be completely lost if it were placed at the beginning of the game.
In short, it's easier on new/ young players to give them grass routes and super-basic battles to start, instead of potentially alienating them with hard-to-grasp puzzles and layered gameplay from the getgo. RPGs always give you the basics to start and then work you into some different stuff -- Pokemon is no different.
It's easy for us to say ice areas are easy, but we're not the only ones playing these games. Contrast Pokemon with a hard RPG like Fire Emblem: they're clearly made with different audiences in mind, and their content is structured relatively.
