Because Sevilla won Manchester United and some people are making quick conclusions that Sevilla will succeed this time in Juventus. Sevilla was able to beat Manchester United because Manchester United up till not in their best form, it doesn't mean they will succeed over Juventus too easily. Juventus is a great team that can't be underrated which I know they will defeat Sevilla. Sevilla performance with juventus will be totally different from the one they had with Manchester united.
In a direct knock-out matches, you don't need to be at their best form to win it. What you actually need is a good (or best) performance in a single match and luckiness of course to beat your opponent. You don't need to be dominantly stronger than your opponent to beat them. In Europa League semi finals, there are two direct matches and a winner must play well and have enough luckiness in both matches. They must do their jobs well first and luckiness can support them to win semi finals.
Sevilla and Juventus have different positions in their domestic leagues but despite of very poor performance in early and middle of season, Sevilla now are at a safe position in La Liga and their form is good enough. While Juventus have a better position in Serie A and they also have a good form lately too, I don't think they are different in quality against Sevilla.
I believe the winner will be decided by very slim differences.