At the high school and sometimes college level, coaches want "bang for their buck", so I think the decline in participation is due more to economics, rather than liability, though I'm sure both factor into the equation. If the schools can't afford to buy equipment, the event sort of fades away. Not sure about the cost/durability of a javalin, but it wasn't an event at the HS level where I grew up.
In college, our coach never recruited pole vaulters. They could score a maximum of 10 points per meet, and they'd break 5 to 10 poles a season at $200+ each. (Saw a guy from another school break 3 poles in one meet.) Just wasn't "cap friendly" for a small school with a limited budget. When we went out of town to meets, we also had a $5 limit on meals, so that tells you how tight our coach was.
My freshman year in HS (25+ years ago) the pole vault pit was wooden palets for the base and big mesh bags of foam piled 3+ feet deep for the padding. Later they splurged for a real pit.