Spurs made three signings, including Mathys Tel and Kevin Danso, both arriving late in the window.
With the squad dealing with an injury crisis, fans had hoped for earlier reinforcements, but Lange pointed to external factors for the delays.
Explaining the situation on SpursPlay, Lange said: “We, as a club, would have liked to have done all the signings on January 1st. We were ready to do that. But, it's not as easy as that.
“This January, in particular, the dynamic changed a little bit. We're not the only team who has injuries. And, there's the dynamic of the new European format.
“Meaning that the clubs competing in Europe have had to play group matches in January, which is never the case.
“Normally, clubs would know their fate in those competitions by December. But the fact is that there are now two group games in the last part of January, as well as more teams going through the group stage into the play-off stages.
“And those things mean clubs are more reluctant. And it's about finding the right players - not just the first players available.”