If you want to hear the full reasons behind the changes, here's an excerp from Races & Classes
In the beginning thieves had the backstab abllity, and it was good. A +4 bonus on attack and double damage were great back in the day, but they came with a catch. It was really really hard to actually complete a backstab. The rules were vague about how they worked, and most DMs shied away from allowing thieves to use this ability on a routine basis.
When D&D 3E arrived on the scene, gamers who loved rogues had reason to celebrate. Sneak attack, whlle perhaps not as swingy as backstab, was clearly implemented and easy to use. Those extra d6s of damage were great. At least, they were great when the rogue got to use them. Entire categories of creatures, mostly notably undeads and constructs, were immune to sneak attack. Wilthout the offensive boost provided by this ability, rogues were severely crippled.
For D&D 4E, we've made sneak allock more flexible while retaining it's basic mechanic. You can now use sneak attack whenever you have combat advantage, a combat modifier gained whenever an opponent's defenses has been compromised. Flanking a foe gives you combat advantage, as do some special abilities. More Important, immunity to sneak attack has been scaled back to almost nothing. Almost every creature a rogue now faces has the requisite vulnerable spot needed for a sneak attack to take place. While a construct might lack Internal organs, you can still smash its knee or find a weak point in its construction to deal a fistful of extra d6's in damage.
This change reflect one of the important philosophies behind D&D 4E. Some abilities are so key to a character's class that they should rarely, If ever, face a blanket immunity. Monsters that shut down one character are more likely to make the game dull for a few characters, or force the spotlight on to a sole character, rather than create interesting situations for the entire party. The rogue relies on sneak attack for his or her offensive abilities, so we're much better of making it a reliable tool.
As a rule, irnmunitlies are almost completely gone from D&D 4E. In their place we have damage thresholds to reflect resistances and invulnerabllity. A fire elemantal might ignore a wizard's fireball but an elder red dragon can still blast it into oblivion with Its breath weapon.
In the design of D&D 4E, the team sought to create a game where a reasonable Dungeon Master could creale a reasonable challenge for everyone at the table. A DM must make a conscious decision to shut down a PC or close off a set of options. For this reason, sneak attack now function against wide variety of monsters.