Because, according to BS/JW eschatology, the second (earthly) calling could only exist in the 'last days.'
Weirdly enough, one major reasoning that led to the 'other sheep/great crowd = earthly' connection was the understanding that Matt 25 (the part about Jesus arriving in his glory with his angels, etc.) was being fulfilled then - from 1914ish onward. The tribulation had been going on since WWI; Jesus had sat on his judgment throne in 1918 and was separating the sheep from the goats based on how they were treating the anointed brothers. Naturally, these nations were on earth while being judged. Couple that with the literal 144k number for the royal/'Christ'/joint sacrificer class and hey presto! (See the August 1935 editions of the WT.)
The Matt 25 interpretation was modified last decade (its fulfillment now future). The 1935 cut off point for the heavenly calling has been ditched only recently. The two class anointed vs. non-anointed doctrine has been seriously undermined: lines blurred.
Edit: The other reasoning Rutherford used in the Aug. 1935 WTs was that (like Oompa said) there were no 2 standards of righteousness as far as Jehovah was concerned so Russell's idea about a heavenly great crowd 'negligent,' also-ran class, who couldn't attain to the high calling, was bogus - and he had a point. Incidentally, previously no one knew who would be of the great crowd class because they wouldn't know until the End who had failed to reach the high calling by being 'negligent.' In contrast, R'ford got to pin down the ID of the great crowd with his new interpretation. But ironically, having thrown out an old 2-tier standard of righteousness, he made a new one in its place!