Simply because Madara is more on a even level with Hashirama now so He's clashing with Hashirama now while still keeping an eye on Obito in case it's time for him to use his trump card. Think about it this way, PS power can be compared to just 1 Bijuu meaning even 1 tails can cause that same amount of damage. Madara said "The Bijuu"
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Madara with PS was still far below Hashirama that he needed the power of Full Kurama to try and win, Kurama (full) is said to have limitless chakra. Ying Kurama, who was out of or low of chakra asked Yin Kurama for some chakra but from his Hands we can see the he was trying to gather more chakra but it wasn't still low because he told Naruto to use his normal form for a while (
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). Upon Yang Kurama borrowing some Chakra, Naruto and Minato joined fist to combined their chakra to give everyone and this is when Hashirama compared it to his own when it was like 45-60% of Kurama's power. (
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Yang Kurama asked Yin Kurama for chakra) and (
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Hashirama compared the chakra to his own).
So Madara pretty much traded Kurama's power for the Rinnegan and Wood Jutsu (Hashirama's cells gave him both plus more stamina). Also Tobirama informed us that Hokages weren't brought back at full power since he said "Almost our full power...." (
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) while Kabuto stated Madara's edo was special and was brought back beyond his prime
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So Madara is more on Hashirama's level now.