It is because you have to use the "kun-yomi" 訓読み rather than the "on-yomi" 音読み for this kanji in this particular context.
Most kanji in Japanese have at least two pronunciations (sometimes more!) - one is the Japanese pronunciation for words of Japanese origin, and there is also one (or more) pronunciation for those words of Chinese origin.
Here, 度 is a word of Japanese origin, so you have to pronounce it using the kun-yomi, so tabi is the right pronunciation.
On the other hand, if you say 毎度, then you use the Chinese pronunciation まいど。
This is a quick rule of thumb (because there are always exceptions): in most cases, if a kanji is used alone (rather than in groups of two or more), it is probably of Japanese origin, and you'll have to use the kun-yomi.
For example: これは、私の物（もの）です。(Japanese pronunciation of the kanji 物）
それは、動物（どうぶつ）です。(Chinese pronunciation of the same kanji)
Of course, as I noted, there are always exceptions: 物語 is pronounced ものがたりbecause even those it has two kanji, 物語 is a compound word of Japanese origin, so you use the Japanese pronunciation.
I hope this was helpful.