[SengalBoy]

It's primarily because they had the costumes around and wanted to put it to use so that it won't be a waste.

You could say part of the reason the Super Hero Taisen movies exist because they need an excuse to use the suits that they reproduced for All Rider VS Dai-Shocker (I believe the suits are reproduced because it has been shown that Toku suits won't last much. Except Sentai suits)

For Ultraman, the same applies but it started way back in 2005 with Ultraman Max. Max was basically Showa Ultraman in modern day so they recreated old kaiju suits, and when Mebius came they reused it as it is fitting for the anniversary.

And later on, Tsuburaya is still recovering financially, which is why the New Generation series tend to feature a lot of classic monsters. Nevertheless, I believe it was from X onwards where they are comfortable with creating new kaijus, but still used the old ones just to have them be of use instead of gathering dust.