I remember Shakugan no Shana getting multiple seasons, as did Ah My Goddess and some others, if we're talking about some of the (relatively) older shows.

It's more the case that older shows got OVA runs instead I think, then had Series 2 as a different name (Full Metal Panic) etc..

It's better nowadays since someone realised how silly trying to sort out each "series" into chronological order was, and it's now just Attack on Titan S1, 2, 3, 4. Sure it has OVAs too.. and see how much of a pain in the ass it is to figure out where that sat in amongst the series?

Without being in the industry, I can only guess that nowadays producers recognise that long running series can be profitable and thus actually plan for multiple seasons, as opposed doing one long run while running into production issues (Naruto) or completing a series then erratically trying to figure out how to release more episodes (OVAs vs a new series).