I'll answer for both :D
HG/SS
Suggestion 1: I presume they didn't include the Poke Radar because of the fact that Gold and Silver never had it.
Suggestion 2: They introduced the Pokewalker.
>The Poké Radar is not normally obtainable in the main game; however, a primitive version of the Poké Radar exists on the bundled Pokéwalker.
Suggestion 3: They made a new grass sprite. It may sound stupid, but they would have to make all the sprites again.
B/W/B2/W2
Suggestion 1: They had just made a new region, new Pokemon and new mechanics. It would take too long to add in the Poke Radar.
Suggestion 2: Because of the new game design, the Poke Radar would be hard to place in.
Suggestion 3: The Poke Radar's most notable use is to increase the probability of encountering the normally extremely rare Shiny Pokémon. Shiny Pokémon were a lot more available in Gen V than IV, as the Masuda Method increased, the Shiny Charm was introduced, which increases the chance of obtaining a Shiny Pokémon, you could capture a Shiny Haxorus in the Nature Preserve and you could get a Shiny Gible/Dratini as a reward for beating Benga.
>The Poké Radar no longer exists in this generation, but the probability of obtaining a Shiny Egg from the Masuda method has increased, from 5/8192 to 6/8192 (approximately 1/1638 to 1/1365).
>In Pokémon Black 2 and White 2, as a reward for seeing every non-event Pokémon in the New Unova Pokédex, the player can receive a Permit that allows them to travel to the Nature Preserve, where they can encounter a level 60 Shiny Haxorus. Also, after defeating Benga in the Black Tower (B2) or White Treehollow (W2), he will give the player a level 5 Shiny Gible (B2) or Shiny Dratini (W2).
>As an additional reward for catching every single non-event Pokémon in the National Pokédex, the player can receive the Shiny Charm from Professor Juniper. Having this item in the Bag increases the chance of obtaining a Shiny Pokémon by a flat 2/8192, resulting in a 3/8192 chance (about 1/2731) of finding one normally (an effectively tripled chance) and an 8/8192 chance (1/1024) of hatching one through the Masuda method.
Hope I helped.
