Star Wars series is the series that re-issued the most... Other than that i got no idea.. 

Most probably is because the set is already EOL which means end of production... So market price went up like rocket... Eg UCS millenium falcon...

Nope! A re-issue of the exact set will not happen. What Lego will do is release a newer updated version with improve building techniques and details of an old set. Probably the most significant one being this year's UCS X-Wing, they also did similar releases with other SW classics like 9492 Tie Fighter, 9493 X-Wing and 7965 Millennium Falcon. I doubt they will do that for franchise/licensed sets but SW being so popular it is the exception rather than the norm...

What sort of old sets are you looking for? BrickLink will be the best source for discontinued sets. Either buy the set from BrickLink or build your own from parts ordered from BrickLink. Try it, it's challenging and fun!

legos don really reissue same set. they will create new variations for SW like red x-wing. but more or less, they look quite similar.

mill falcon i think they wont reissue. because there's no other ships in SW universe that looks like mill falcon.

Gasp! I'm just ikan bilis la with my very limited knowledge! I'm what they call "Katak Dibawah Tempurung"... only small fry in the Lego universe.

No point chasing those sets, prices are high and if you work out price per minifig it is very high! Honestly the new SW minifigs from year 2012 onwards are quite impressive so much so it makes the old minifigs rather boring.... So really you are not too late in the game, start collecting now! haha!

Just some add add on, if you're into Super Heroes theme.

The old Batman theme has been "reissued" under the DC Superheroes banner.

Unfortunately, the new sets are smaller and less "wow". Best comparison is to look at the Batcave and Batwing sets.

Costume for minifigs (or Batman at least) are now more comic style compared to the 2008 sets which had more sets with "The Dark Knight" suit printing on the minifig.