There are a lot of variables that could have taken the parks shrubbery and I'm in no way saying that any of this happened, but here are some things that COULD have happened:
1. Extreme cold and/or Heat can damage or kill plants
2. People walking on or around the bushes can cause broken limbs which can in turn could kill part of a bush
3. Disease, once it starts it can spread quickly. Like last winter, we didn't really have a frost nor did we get a lot of rain so if you remember the pollen was almost intolerable last spring, because the cold/frost didn't knock the spores down like normal. Also, remember that people come from all over the country and can bring disease and/or fungus spores on their clothes.
4. Improper trimming, if bushes are trimmed too far and/or at the wrong time of year it can kill parts of or the whole plant
5. Too much water or not enough water can kill plants
Those are just a few reasons that come to mind. Along with those I think it needs to be said that this is not your home flower beds, things done at the park are going to be done by the book, so if something is half broken it will probably be removed and replaced(OR not replaced because of budget), it's a whole different level of landscaping. My guess is they would rather have a dead spot than a dead Busch. (<- see what I did there
)
I also want to say that sometimes bushes just get too big, even if your trim the outside new growth, the trunks continue to grow and over the years they may get too large for the space and have to be removed and replaced with small versions. This happens a lot with certain trees. Actually I was in LoD over by the dragon ride and if you look just over the edge you'll see 2-3 Magnolia trees, these trees when mature can span 20ftX20ft and will not be good for that area, BUT they grow extremely slow so they wouldn't reach that height for 30+ years which is more than enough time to remove them before getting too big. As wasteful as it sounds, this is pretty common.
Lastly, when thinking about why the plants aren't growing, you need to worry about the nighttime temps rather than the daytime temps. Until the nights are reaching upper 40's into the 50's most things will remain dormant. Many of the pictures posted by BGW Family were of tulips, those are bulbs and will not sprout until it warms up. Much of the color he posted were all pansies, which are throughout the park now.
I can't speak to how the park used to be and how much if any has been removed because I didn't visit that much in the past, but personally I think the park looks great. If it does have less landscaping, my guess would be that some had to be removed for whatever the reason and they just didn't have the funds to replace them like in the past.