Why is the past tense of text, as used by some people, pronounced text-Ted and not just tested? One wouldn't say risk-ked for risked, or ask-ked for asked?
The final t in text makes all the difference. In risk and ask, you are ending on a final s followed by a k sound, so you are able to add the d without trouble. But in text, you don't have s followed by k—you have k followed by s followed by t. Not only are the s and k sounds swapped, but you have an extra dental sound to finish the word. There is no clean way to transition from kst to a d sound the way that you can in the word ask.
There is a way to distinguish ask from asked (when hearing the words). There is no way to clearly distinguish text from textd—especially in American English where the t and d sounds at the ends of words are not always distinguishable.
It's also worth noting that using text as a verb is a recent construction, so the etymology of the past tense of the verb text would be based on what "sounds good" to modern speakers rather than what "sounded good" in the past when ask and risk were first used in the past tense. In other words, when dealing with new construction, one can't always rely upon how it used to be done as evidence of how it will be done in the future.
It may have something to do with the "xt" in text being a 3-consonant cluster since in English,the letter x is pronounced "ks". Thus another phonetic spelling would be "teksted". The difference in pronunciation seems to be because the syllables can be divided as either "tek-sted" or "teks-ted".
The "sk" in asked and risked is only a 2-consonant cluster; therefore, the division between syllables can only occur one way.