Because it's not a passing chord. The type of 6/4 record refers to what the bass is doing. In this case, the E in the bass isn't a "passing" tone (i.e., it's not approached by step and left by step in the same direction), so it isn't a passing 6/4 chord. Now, the third chord in m. 3 is technically a correct passing chord, but odd because it's minor (although minor v's aren't uncommon passing chords) and the aforementioned poor voice spacing.
Minor v chords definitely exist, but—in America, at least—you're just taught to always raise the leading tone in minor unless otherwise marked through figured bass. V would indeed have a B-natural, but your use of the minor v, at least where I teach, in and of itself would be suspect.
Any diminished or augmented interval in the upper voices is forbidden (unless extremely niche circumstances I won't cover here). If you did make the alto a B-natural in m. 3, it would become an augmented fifth, which is augmented, so it's not permitted.
I mean, I'm sure there's music that ends in IV like some sort of plagal half cadence, but for harmony exercises, we really only deal with Half Cadences, Perfect Authentic Cadences, and Imperfect Authentic Cadences, with plagal extensions and deceptive cadences as ways to lengthen a progression, not end one. Ending a phrase in IV is really odd because it's functioning as predominant, which implies there should be a dominant somewhere in there.
Note your wording. Authentic cadences resolve to tonic. Half cadences merely end on the dominant. Most phrases and pieces end on tonic because they "feel" resolved.
I'm not explaining it in detail here, but 6/4 chords were considered the most unstable because of a remnant from Renaissance-style practice where creating a fourth between the lowest sounding note and any upper voice was a huge problem. So we only use passing, neighbor, pedal, and cadential 6/4s because 6/4 chords are dissonances and need to be treated as such. The fact that you have two 6/4 chords in a row in m. 3 is wrong because you have two dissonant chords in a row, prevents proper resolution.
To be clear, all of this is only true in terms of doing exercises. Exceptions in the repertoire exist all the time.