Why wasn't there an upgrade to R2-D2?

I was watching Star Wars Episode I (The Phantom Menace) with my sons yesterday. As R2-D2 was being lifted into Obi-Wan Kenobi's ship, I wondered why in Episodes IV - VI they were still using the R2 unit technology for the X-wing fighters. If you think about it - Episode IV (A New Hope) takes place at least 20 years after Anakin hooks up with Padmé to produce Luke Skywalker. With the technology everyone has in Star Wars, don't you think The Rebel Alliance could've upgraded the R2 technology? Maybe one that speaks English instead of throwing out a bunch of beeping noises that requires another droid to interpret. Perhaps an upgrade to its mobility â?? like some legs that have knee joints. It's just not practical to have a droid who's going to be dodging gunfire to require a smooth surface to roll around on. Don't get me wrong, I love the Star Wars series, I just think George Lucas should have thought these things through a bit.