I think this is mainly because we could be judging from the performance of Tottenham last season and looking at the fact that except for Manchester United and a few others, most of the teams qualified for the Europa aren't coming from a very competitive domestic league. Manchester United is supposed to be the biggest rivals alongside Tottenham but their performance so far hasn't been really encouraging enough to see them in that regards,  when Manchester United started off the season with a victory in their first game, it aroused some beliefs they could get better by the season but not long after they are displaying weakness already and this is in their domestic league so it raises doubts if they are going to perform well in the Europa.
Manchester United? Favorites for the Europa League? You've got to be kidding me. They're a mess. Ten Hag's tactics are all over the place, and the players look lost. Ugarte's a decent signing, but he's not going to fix their fundamental problems

Spurs, on the other hand, have a history of showing up in Europe. Postecoglou's got them playing a new style, and it's not perfect yet. But they've got that intangible quality; they know how to grind out results when it matters. The Europa League is wide open. Any team can get hot and make a run. But if you're looking at the favorites, Spurs are the clear choice. United? They're lucky to be in this competition. They need to focus on getting their own house in order before they start dreaming of European glory