The question of what type of spell to use and why probably goes back to the beginning of witchcraft- anytime we are presented with options, the choice becomes both a blessing and a curse. I understand your curiosity, @mary25! 
You’re right that there’s no one answer here. Instead, I think there are few possible answers as to why and how one might choose a type of spellwork.
Situation: Let’s take a look at @Silverbear, who is the Queen of Witch Bottles- she excels at them. She is blessed with a wonderful working space and lots of amazing materials on hand. I, however, almost never make jar spells- as a witch on the road, they take up way too much room in my suitcase 
. So while a jar spell is a great option for some people, it’s not an option or at least not a good option for others. The materials/ingredients and space you have to work with can all heavily influence your spell decisions.
Knowledge: One look at the list of weekly witchy challenges will attest that there are so many areas of magick out there. Whether new to the Craft or experienced, we all have varying areas of knowledge based on our personal research and life experiences. Someone who would be comfortable with a candle spell may shy away from sigil making as they know nothing about it. I think it’s natural to fall back on what we know and are confident with, and that can certainly affect choosing what type of spell to cast 
Preference: Every witch has types of spellwork they like- and many of us have things we don’t particularly enjoy. I tend to avoid things that involve angels, numerology, and palmistry- not because I have anything against these areas, simply because they just don’t appeal to me 
So if I was looking for a love spell, I would likely skip over prayers to angels or reading palms and choose a spell with a candle or a tea reading instead. Your likes and dislikes play a big role in choosing spell types.
Situation, knowledge, and personal preferences all significantly narrow down the pool of options and make it easier to choose a type of spell. From there I think it’s safe to say that intuition kicks in- once a caster has narrowed down their options, they will usually go with a spell that feels right or calls to them.
Reading the spell instructions closely and making sure that the particular spell is what they are looking for can also help to pinpoint exactly the right spell for them.
I’m not sure if this helped to shine a bit more light on the hows and whys of choosing spell types, but it certainly helped me to organize my thoughts on the question! 
Thanks for brining up this discussion, @Mary25, and I hope you are able to find the answers you seek! 
Blessed be 