We've seen the likes of Portsmouth all win cups in recent years, proving you dont have to spend big money to win a cup.
Pompey have been in financial meltdown because of what they spent to win that cup!
Overall there are obviously a few reasons but the biggest for me is it just really difficult. Liverpool 1 trophy in 9 years (league cup playing a lower league team in the final), arsenal 1 in 10 (playing a lower league team and hull in the last 2 rounds). They are 2 of the best and most consistent teams over the period and have 2 trophies between them. Sometimes the stars align like Wigan getting us on a meltdown day and city when the manager was known to be sacked regardless, Pompey only playing 1 premier league team in their run man united who had their keeper sent off, swansea getting a league 2 team in the final, whereas us in 2009 had to play 1st, 2nd, 4th and 6th and we were the 5th best team!
Yes more money would help, yes more bottle from moyes, less naivety from Martinez in Kiev gives you a better chance and if you combine all of them then you increase your chances. But even with all that it is really hard unless you have city or Chelsea money before FFP. After all man united haven't won an FA Cup for 11 years now.